Board Thread:General Discussion/@comment-4926663-20130829025922/@comment-12053-20130916041852

With pure honesty, I think Slash would start of as an friend but quickly proven to more antagonistic than Raph can imagine. Raph stubbornly tries to make it work for everyone but things goes out of the line. With everyone realizing that Slash is not the hero-material (including Raph and Slash himself) so Slash picked a different route which suits him best; a life of destruction.

That and the route of enemy-turned-good has already been explored with Mike and Leatherhead. And we need a proper villainous rival for Raph.