So why do we end up invariably feeling like Monte Cristo is in the right and the rest are in the wrong if they've all committed crimes?

Because he's so likeable?

For me, it is probably because he was plotted against in the beginning and quickly gained my sympathy. The plotters gained my distrust and enmity. I don't think I could ever have come around to having sympathy for them. But I was pulling for Dantes from the beginning, first to get out of prison, then . . . . etc.

I think Dagny is right. We are immediately sympathetic towards Monte Criso because of what is done to him, we sympathise with his wish for revenge - I mean havn't we all wanted revenge for something at some time or another. As a character he is intensely human, with problems, feelings and motivations we can all empathise with. The plotters on the other hand represent everything we are against and again we can relate to them on the level of we would want bad things to happen to them if we were Monte Cristo.