Recap / Teen Titans S 3 E 9 The Beast Within

Adonis: What's the matter, wimp? Don't you know how to fight like a man?
Beast Boy: I'm not a man ... I'm an animal!

Beast Boy gets drenched in chemicals while fighting the villain Adonis, after which his behavior starts to change dramatically.

Tropes:

http://tvtropes.org/pmwiki/pmwiki.php/Recap/TeenTitansS3E9TheBeastWithin