When he executed Baxter, who was already completely defenseless. And hey, I do think Mirage Donatello is more or less a good person; he's just done evil things before. Baxter pretty much pushed him to it (largely because Baxter was trying to keep Donnie busy instead of working on a way to save April because Baxter wanted April to die), but it was still a very dark thing to do. For the most part, Mirage Donnie was never allowed to be too evil, since he was Peter Laird's author avatar and creator's pet.

If you want a version of Donatello who has succumbed to evil, try Donatello Hamato from MNT Gaiden (linked off-wiki). Though even he is not completely irredeemable, as he's also struggling with mental illness.

Most other versions of Donatello I can think of are...more or less benign, with a notable exception being Metal-Don, and even his darkness grew out of the darkness Donatello already had. But whereas Donnie can't just completely detach from his emotions and become a complete sociopath, all Metal-Don had to do to escape the agony was delete his own robot emotions, and hello Donatello sociopath.

So, you're right, there's always potential for a character like Donatello to become evil, circumstances permitting. Though considering my original example was Mirage Donnie, it's important to mention that Mirage Mikey ended up doing much, much worse after a perfect storm of the worst things happening to him. So...yeah.

OK,so...you do mean Baxter! Well, this is my problem : i always point out the bad things and the good things BUT everybody complaint when i point out the bad and when i point out the good,they tell me that not everything is good!!! X | =_= Let's start, we all have a "evil side",i know,it's just that...if we think of Mike or Raph,they're good guys (i can't say the same of Leo,but that's other theme) but Don,he accumulates grudges,that's the bad side,i know he fight to don't give in to his "demons" and that's the good side. Now, some examples of poor evil Donnie :

1987 serie episode :"Donatello's duplicated" Well,here his duplicated was IMPROVED was NOT a EVIL CLON just improved,ALTHOUGH,he wanted to CONQUER AND  THE WORLD,can you explain me that,please???.

1987 serie episode : "Night of the Dark Turtle".here did NOT changed personality,he just "drew his resentment" with Shredder and he "broke" and nothing more... but just because he is patient,he hadn't done anything...yet.

2003 serie episode:...(i can´t remember) but all the seventh season he was angry (about Splinter,of course,and his brothers were irresponsable,but REALLY MAD)

2003 serie : "donatello's clon" do you remember that guy? Oh,i bet yes! well he's just his "evil side" and he has the same personality as his 1987 clon. Curious,doesn't it?

2012 serie episode :"Heart of Evil" and "I Think his name is Baxter Stockman" (in the trivia of Donatello say this : "Due to his behavior in I Think His Name is Baxter Stockman (and various other episodes), he is the second to Raphael in temper and anger."

Image comics : "Don-cyborg" (He scared me) He even almost fight with Leonardo.

Mirage Studios : your example and many (MANY) things. Anyways, he didn't wanna kill Skonk (from C.R.A.P. from "Survivalists) And i feel that he's "mysterious" and serious,just that.

IDW comics : "Metal-Don" (poor) actually, i don't know a lot of IDW... i haven't read it :V i begining with Archie Comics but i'll do it ¡i can assure it to you!!

By last : MNT Gaiden i already know that comic...¡it's wonderful! ¡i LOVE it! however... i'm SORRY for him,he i pity him :( many say he's gonna die or something like that but i DON'T think that *sigh* i'm glad you think it's not irremediable.

And i don't think Mike has become evil or vegeful

Oh, really?

Mikey really needs a splintervention.

2012 Donatello is the biggest jerk of any incarnation of the character to me. But he was raised by a Splinter who is a jerk himself at times.

Yeah, I've written a few fanscreenplay moments to that effect. :)

DONNIE
So, Raph gets teased about Slash because he tries so hard to deny it.
But Mikey never gives anyone a reason to tease him.

Mikey stops work and faces Donnie.

MIKEY
What?
(angrily)
You guys tease me all the time!

DONNIE
For myriad _other_ reasons, Mikey.

MIKEY
What's "myriad?"

DONNIE
Reason number ten thousand.

MIKEY
You've been counting?!

DONNIE
_And_ ten thousand one.

Also (because these are fun):

MIKEY
(nervously twiddles fingers)
It's easier with pizza.

LEO
We're all dealing with this in our own way.

DONNIE
Mikey's way is to shovel greasy Italian food in his face and somehow never get fat.

MIKEY
(faces)
It's my gift, brah!

DONNIE
(faces, deadpan)
Mikey, that's not a gift.
That's a black hole growing inside you.
You could be one pizza away from killing us all.

Leatherhead CHUCKLES.  Mikey scowls at Donnie.

Yes,yes i know that,i already see that image,i read the comic,i know what she did him and how many worlds he destroyed with his new friends;the triceratons! it's just that...an "evil Michaelangelo" don't fit into my head! What i was gonna write was (i couldn't even finish that writing,by the way) : i don't think Mike has become evil or vegeful... i think he underwent a personality change of his character; To you understand me better see Mark Pellegrini's : TMNT Vol. 4 #32 (review) ... thank you! XD :( And I had trouble writing all that about Donatello, so can you tell me what you think of what I wrote about Don, please ... FORGETING MIKE? Oh, and i've not finished with Donnie; There are two more stories, but they're on Deviantart, though, they're good and describe exactly what I mean! XD (but more so the second) : first story : 

Dr. Don - Chapter 1 by Lokrume on DeviantArt (Chapter 1 of 12)

And second story : 

Slumber's Torture: C1: Just A Nightmare - YAYProductions - DeviantAr

(Chapter 1 of 14) this one continues in its next saga :

The Monster Inside: Chapter One: His Torment Began (geez! what a name!)

The Monster Inside: Chapter Two: If It Killed Him

The Monster Inside: Chapter Three: To Destroy His Family

The Monster Inside: C4: Everything Went Black by YAYProductions on

these are the first four chapters but you can continue reading it if you press to the bottom of the page where it says : The Monster Inside (and NO,this link doesn't work,just do what i tell you,OK? XD)

And this continues in here : MNT Kamegatari- SAINW Origins YAYProductions en DeviantArt

(see the chapters' numbers to don't confuse you)

And this continues in here : yayproductions.deviantart.com/...  (this only has three chapters, unfortunately, they haven't finished but that doesn't matter, it's NOT NECESSARY because If you want you can finish at the end of any saga)

And Avatar Beta is right XP this silly and nerdy version of Don is my least favorite, actually this Splinter is my favorite of all versions but he has REALLY been an idiot at times (i hated him in Tale of the Yokai) Anyways, i liked your "fanscreenplay moments" (and "Splintervention") ^^ _ :D

OH,YEAH... I HAVE FINISHED!!! (I'm soooo happy!) 

I'll wait for an English translation, for everyone else's benefit. Google Translate can only be so accurate anyway.

¡READY! ¡READ IT NOW PLEASE! XD XD XP (I didn't know what myriad was either XP)

Yeah, that episode really put him in a bad light for me. They say say Shredder lies to himself to justify hating Splinter, but from the looks of things, Splinter was just as bad as him when it came to accepting his own faults.

I haven't watched most of the 2003 TV series, and though I know Same As It Never Was is from that series, that doesn't actually mean a whole lot to me, let alone fanfiction inspired by it.

Also, while I find Donatello having an occasional dark side somewhat interesting in terms of character development, and I do sometimes read novel-format fan fiction of various kinds, I generally don't read very dark or creepy fiction written in novel format. When it comes to bodies of text, I tend to skim first, and then read in depth. But if the non-dialogue text is too florid in its prose, I'm not usually actually carried by the feeling it's trying to convey—rather, it's one more noisy layer to sift through. So, when it tries to be really dark, that...doesn't always translate well for me.

I prefer screenwriting anyway because you can focus on scene description and dialogue, inspiring the reader to imagine finished production in their mind while reinforcing the general rule of "show, don't tell." I mean, you can leave notes to assist in production in various ways, but the end result is something that can only be seen and heard, so you want to make sure the audience sees and hears what you want them to.

I have tons more fan screenplay moments involving Donatello, with him either being acid, or being taken down a notch, or both, though all of them are in stories where he's mainly a supporting character. My binge-screenwriting phase came at a time when I was increasingly frustrated that the 2012 TV series went from showing lots of subtle character development and banter, to increasingly showing little to none at all. The first two seasons were jam-packed with that kind of gradually-built detail, and alone seemed to promise a show that would only get better in the drama department. Yet in season 3 onward, writers seemed to get really, really lazy, and character interaction became more wooden and rubbery compared to the zest and bite of earlier seasons, with the result that characters lost a sense of depth. It's most fun to see characters like Donatello interact in an environment of baggage-lain subtle characters, where the things they say to each other have more context and more flavor. Maybe someday I'll publish what I've written, but it's still unfinished after two years, and it's gone from "TV-Y7, plausible in-series" to "TV-14 or TV-MA, must exist in an alternate universe", a change which only accelerated after the character derailment sideshow that was season 4.