When I think "bloodthirsty" someone like Nicky Scarfo comes to mind.
For the most part,Mike had justifiable reasons to kill most of his victims.
Some exceptions that come to mind are the Lake Tahoe prostitute,and Roth.
The girl was a truly innocent victim,and Roth was finished.
I think that his killing as well as Fredo's were done both to satisfy Mike's ego,and to (in his mind) show the Mafia world that nobody messes with Don Michael and escapes vengeance.