Yeah, I agree. Cao Cao does bad things because he has to as a leader.
Sun Jian seemed okay. He did technically "steal" the imperial seal, but he did find it abandoned in Luo Yang. It's not like he intentionally stormed the palace and killed the guards just to steal it or anything. He thought it was a sign from the heavens, too, so it's understandable why he'd keep it. He died too early for me to really judge him though. Sun Quan on the other hand, I really despise.
I'd still say Liu Bei's much more virtuous than Cao Cao, but he's not completely pure either. He was the one who initiated the conflict between himself and Cao Cao. At the beginning (up until Chi Bi, I think), Cao Cao greatly respected Liu Bei and wanted to work together with him. But then again, Liu Bei only separated himself from Cao Cao because he realized that Cao Cao wasn't such a good guy and wouldn't be able to create the land of virtue he dreamed of. As a ruler in general, Liu Bei wasn't really great, though. I think it was his advisors (mainly Zhuge Liang) that kept Shu strong. I hate to say it, but during war, I think one has to be cold-hearted and objective sometimes.
Cao Cao wasn't such a nice guy, but he was a very firm and good leader. Liu Bei was a very nice guy, but he was only an average leader, nothing remarkable. I'd say Sun Quan was about an average a leader as Liu Bei, as his father and brother already did a lot of the work for him, and he did just as many bad things as Cao Cao did, which is why I don't like him.
"There are five possible operations for any army. If you can fight, fight; if you cannot fight, defend; if you cannot defend, flee; if you cannot flee, surrender; if you cannot surrender, die. "