Though in all likelihood I do not think he could kill Cao Cao. Mengde survived a hell of a lot and really he was not an easy target to kill.
Wrong. I quote Michael Corleone from the Godfather II. "If there is anything history has taught us. It is that you can kill anyone."
Cao Cao was no more suseptible to being killed than Liu Bei or some other persons during the 3k era.
However not to stray from the topic. If Cao Cao had spared Liu Bei, there is no doubt that Cao Cao would be a dead man. Even if Cao Cao constantly ordered Lu Bu around, Lu Bu would finally sieze the opportunity and seek revenge. Lu Bu is a man who sees no integrity in honor and loyalty. He reminds me of Akuma from Street Fighter. Kill those who are weak and then kill some more. Plus Lu Bu could have easily taken on any of Cao Cao's generals.