Why he should stay a villain
Slash could be a fine villain. Besides the emotional connection which would cause all kinds of conflict for our hero, he knows all of Raph's secrets. Think about it! It's like if your diary suddenly came to life and knew all the darkest moments that you wrote about, things that no one else was ever to hear, things that were said in anger, bitterness or moments of despair, things you got over and felt better for just getting the chance to vent. And that's all you needed. But now, this thing; this monstrously HUGE, powerful thing, knows it all. All your dreams, all your sorrow, all your hurts and all your shame. It knows them ALL. And can use them against you. It can push the buttons that you have buried deep in your heart. Secret jealousies, longings, EVERYTHING! What greater weapon is there than to know first hand the hidden corners, the protected corridors, and the shameful yearnings of your foe's heart?! Ah, I lament the loss of such a most excellent possibility for a villain. They set it up so carefully, only to throw the potential right out the window. Sigh. Thank goodness for fanfiction. I'm gonna have to revisit my buddy Slash soon.