You have not been cruel to Oliver, you have done what no one did before - you got to know him, you tried to help him, and you loved loved loved him. Now, when he is at the shelterm it will be with a note that he needs to be an only cat. Think how happy he will be when that actually happens!

Some kitties (and some people, too) just don't play well with others of their species. Oliver's one! It's just part of what makes him him, just like his shiny fur and pretty eyes.