2 kills within about 2 seconds=Double Kill
3=Multi Kill
4=Ultra Kill
5 and beyond=M-M-M-M-MONTSER KILL Kill kill...

Killing Spree, Dominating, Unstoppable and Godlike are what you get for a bunch of kills without getting killed. It's pretty easy to get Godlike on certain levels against bots if you camp in the right place.