Board Thread:Questions and Answers/@comment-32442791-20170714193508/@comment-995426-20170715070958

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 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, 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 ended up doing much, much worse after a perfect storm of the worst things happening to him. So...yeah.