Out for Justice (1991)
'He's a cop. It's a dirty job... but somebody's got to take out the garbage.'

Here Seagal plays a cop from Brooklyn whose best friend is killed. Seagal then hunts down his friend's killer and uh yeah, that's pretty much it. As you can see the title also fits the formula and the poster features Seagal holding a gun, though the shotgun is a bit of a gamble compared to Seagal's usual handgun, but I think it works well here. Also I forgot to mention that his friend's killer is a drug dealer.