Oliver Reed was a pretty good actor before he decided being a clown was an easier option. His best performances have a sort of brutal sensitivity: he's vulnerable, and liable to smash your face in if you mention it. In 'Sitting Target' he takes this persona to the very edge.
Harry Lomart is a professional criminal who can endure any hardship inside as long as he has the backing of his much loved wife outside. When she tells him that she's leaving him, he breaks out of prison to kill her, her lover, and anyone else who gets in his way. At the end of his rampage, he simply lays down to die besides the body of the woman he loves, all passion spent, all point gone.
It's a pretty hard hitting film, and Reed is brilliant in it, a force of nature, a brute with a broken heart. Lomart is no cold, ruthless Jack Carter, he's full of rage and barely in control. That said, the net result is more or less the same. Revenge, eh?