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".