Games with a lot of violence

Manhunt is alright. Pretty good stealth game, but it's VERY hard in my opinion.

Serious SAM

Saints Row 3, 4. Gta 4 and so on. Prototypes are also pretty good.

Saints Row IV. Prototype 2.

Oh. Since you changed the title, Serious Sam and Duke Nukem.

Secret of the Magic Crystal

Pacman

Attention Wii owners: MadWorld. That is all.

Total Overdose.

If you own a Wii you can play Madworld.
I don't know if you like these kind of games but you can try "Super house of dead ninjas" and "Warframe". They are both rather violent with a lot of ennemies.

Just Cause 2....$3 on steam right now....so violent and explody, its wonderful

Trouble in Terrorist town

carmageddon

Warframe (F2P on Steam). The way you execute enemies, brutal stealth kills, gore deaths (different with weapon types/elemental damage).

Also, Hotline Miami!

It's not free roam, but if you want bloody violence, how about Bulletstorm? :)
(To clarify because the screenshots show jack shit- you can shoot a guy in the balls and while he's screaming in agony kick his head off his shoulders... YES.)

Brutal Doom, Mortal Kombat 9, enough said

The Darkness II, and +1 for Hotline Miami
Manhunt is technically violent, but it's also kinda boring. I can't even remember if I've finished it or not.

Darkness II

Doom

BloodRayne 1 and 2

Tetris

Bulletstorm

Bulletstorm and Bloodrayne

Rise of the Triad

Manhunt with the lights off and correct darkness setting. Sadism, ho.

I'm surprised no one has mentioned Chivalry Medieval Warfare.