Heartwork: The Game

Yuu Asakura was a mild mannered university student until he found a mysterious duffel bag with a gun and orders for an assassination. He was terrified at first, but when he held the smooth metal of the pistol in his hand, he started to become someone else. Capable of anything, Yuu takes the assassination job and enters a world of nonstop sex, blood and guns. When he returns to school with his newfound abilities, he finds it no trouble at all to get any girl he wants. The syndicate and the police want a piece of the new guy, but the one Yuu has to worry about is the gun's original owner.

[From Kitty Media]