Hitman 2

As a titular hitman, you have to complete a number of missions all over the world. Each mission generally involves assassinating someone and possibly fulfilling a couple of other objectives. Typically, your target will be protected by numerous guards. While there's a large arsenal of weapons available to you - ranging from stealthy weapons such as a garrote or a silenced pistol, up to machine guns and sniper rifles - your main weapon will be stealth.

Agent 47 can sneak around to remain undetected, and he can disguise himself in clothes from killed or stunned guards. The guards aren't completely stupid, however, as they will investigate suspicious noises, and they'll react if they see you waving around a gun. They'll get suspicious if they see you running around, raise alarm upon finding a corpse, and if you hang in a disguise around them too long, they'll eventually realize you're not one of their buddies.