Holy fuck is this game good. I can't believe I waited so long to finally pick it up and play it.
If you want a better understanding of Overkill, well, here you go:
(oh-ver-kil)
n.
1. Destructive nuclear capacity exceeding the amount needed to destroy an enemy.
2. Excessive killing.
3. An excess of what is necessary or appropriate for a particular end
Anyway, in this motherfucking game you shoot zombies. Oh wait, it's key to never use the Z word. I'll call them un-dead from now on. Anyway, you shoot those bastards light-gun style. A headshot makes them explode. Arms makes them 50% dead. Legs slows them down, but doesn't kill them. Stomach's good depending on how much of a fatass the un-dead is. Nuts is ineffective.

You play as Agent G. If you're wondering what that motherfucking G stands for, well ... it's top secret. Sorry. You're joined by Isaac Washington. Mess with him, and he'll rip your motherfucking balls off. You'll also come across Varla Guns, a gun-totting hottie who's out for revenge because of her dead brother or some shit. But, wait, Varla Guns? What the fuck kind of name is that?
You've got a variety of levels, each is made to look and feel like a shitty horror movie. With a poster, and a retarded narrator to boot. For instance, Ballistic Trauma, is a level/movie where you shoot un-dead doctors and patients in a hospital. You're gonna be amputating some motherfucking heads. I guess that makes you a doctor?

You start off with your regular pistol, but you've got plenty of weapons to unlock. Everything from a machine gun, a shotgun, you know, classic stuff. You've got infinite bullets for all of them. You can go with two weapons into a level and switch between them. Shoot with B, reload by waggle. Very intuitive.
At the end of every movie you've got a crazy ass son of a bitch boss to fight. The strategy to defeating them is always figuring out their weak spot (which is shown to you before and during the fight) and shooting the shit out of them.

Overkill's pretty short, but it's great for multiple play-throughs. I personally don't regret buying it. If you generally don't enjoy these types of games, then it's a rent. But
everyone should experience this bitch. You're playing alone, it's very likely you'll have fun. You're playing with a friend, it's 100% guaranteed.
You might say, the graphics are bad. Well, fuck you.
EDIT: Oh, I should also note, if you want, instead of playing with a friend, you can go dual-wielding style, bitches.
I wanted this review to kind of give the feel you're gonna get while playing the gameURL to share (right click and copy)