they were going for a play on words, not something badass. also the "overkill" thing was a joke, because earlier in the episode, ben changes into swampfire and lights a campfire with a fireball.
ben: SWAMPFIRE!! gwen: ben, thats overkill. ben: no, its swampfire. i dont have an alien named overkill or id, you know, shout "OVERKILL!!". kevin: whats with the shouting names thing anyway? ben: i dont know. it strikes fear into the hearts of my enemies.
so later, ben named his merlinisapien form "overkill".
I really loved that episode, and i especially felt sorry for that prisoner. When ever i heard him say: "I want to be with them!" when he refers to his deceased family, my heart aches! and the overkill joke in the episode is just one of the best damn jokes on the show!