The character known as Overtkill in today's review from Spawn Series 1 started life as a character named "Overkill," created live (on video) by Rob Liefeld and Todd McFarlane, under direction from Stan "The Man" Lee:
That's from the Comic Book Greats VHS series, which was advertised in the comics all the time back then. Listen to how he makes fun of them!
Still doesn't explain why the name had to be changed to OverTkill, though.
Someone else had a trademark on the name Overkill- so as many Transformers have, Overkill had to be renamed into something they could trademark.
But that only applies to merchandise--a secondary character in a comic can be called anything. Okay, not "anything," b/c it's not like Spawn could fight a guy names Batman or Spiderman without getting sued, but "Overkill" is so generic that no one could complain about it. Batman has a foe named The Scarecrow and Spiderman has a foe named The Scarecrow and they both exist independently with no trouble.
Ronnie could still be right: it's possible that Todd knew he was going to turn the character into a toy, and, not wanting to deal with name issues when that happened, changed the name preemptively...
That seems likely, considering that McFarlane was making his own toys, not licensing it out. Making the character easier to toy would be in his best interests.