Most of the movie it is not that violent but in the end it gets violent and bloody. But it's not worse than like, for example, Pulp Fiction or Heat (1995). Thief is very, very tame today.
A man is shot with a shotgun. Lots of blood And chunks of meat is seen. (This death is very graphic)
A man is shot a few times in the chest and than the head (lots of blood squirts out)
A man is shot by another man (blood squirts out of the wounds)
A man is shot from a distance (no blood is shown)
Two police pull a man over, and make him get out of his car at gunpoint.
Several police officers beat up a man in a holding area.
Very little to no violence until the finale, which features about 4 bloody deaths. Other than that the film is pretty tame
Frank is threatned by his really pissed-off boss in a car workshop in one scene, in which he describes what he will do to Frank if he keep refusing to do his 'job.
A long scene of explicitly-stated, threatened violence including the gang-rape of a character's wife and the cannibalization of his family in front of him (both threatened, but described vividly).
James Caans Character (Frank) Is Seen Stabbing A Guard At The Bank.
Several violent scenes showing an abundance of blood spray.