Rather than defining him as bloodthirsty or somebody liking violence for the sake of it, I would say he simply didn't care about anything other than his ego; he was a cold as a fish individual (no offense to fishes). Michael's whole attitude towards other living beings could be defined by the phrase "I don't give a fuck", imo. Somebody could argue he did care about his father since he committed his first 2 murders to save him, but the fact he never stopped his life of crime later (even accepting to become the boss of the Corleone family), despite having more occasions to get out, proves there was something wrong with him from the beginning, in my opinion.