Off the top of my head: Sulla, Cromwell, Robespierre, Napolean, Hitler, Mugabe. There are many more, but I'd have to verify.
Where could I find a list of terrible tyrants that came to power in a representative democratic system? That ought to be interesting.
Well, that list mostly goes:
1. Hitler
2. A few others
But, y'know, Hitler counts for something.
Or are you saying that all systems with parties are abominable because of parties, regardless of the details of how the systems work?