It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

In some circumstances when you kill an enemy you get "Overkill" as message. Does it have some kind of function or it is just for glory and gory satisfaction?
I think it's just for glory.
It's a retro thing. Some classic FPS games would reward you if you slaughtered an enemy with already low HP. Another game from the Heretic/Hexen era called it LUDICROUS GIBS