At a certain point after Gru goes to work for the Anti-Villain League, Kevin and most of the other minions are gradually stolen right out from under him, each under mysterious circumstances.  After being temporarily held in an artificial tropical setting, complete with all the trimmings, they are each transformed into evil monstrosities through injection of the PX-41 serum.  Of all the minions, Kevin becomes the second to be transformed (immediately after Tom) and then the first to be set loose, whereupon he makes a beeline for Gru's house, causing some momentary chaos before Dr. Nefario injects him with an antidote of his own making.

(The evil minions are rather more dimwitted than the normal yellow minions who work for Gru.  They also have an underslung jaw with a heavy underbite, over-sized fangs, massive unkempt hair [and a generally hairier body], scraggly chin hair, and oversized, gangly arms.  They cannot communicate in the normal minion language, though they appear to understand it.  Their most frequent utterance is a short, abrasive and intimidating "BAAH!".  They constantly twitch and shudder, and mumble and grunt.  They are also practically invulnerable to harm (even from themselves).  They can be shot by gunfire, exploded by bombs, have massive heavy items dropped on them, flames hurled at them, and still more, and yet bounce right back with no ill effects.  They can also eat anything, regardless of its composition...even explosives.  Their only motivation seems to wreak havoc.  The evil transformed minions were, it turns out, created by Dr. Nefario, when he briefly went to work for supervillain El Macho...until being saved by him when he had a change of heart and returned to work for Gru.  All of them returned to normal...apparently except one!)