I agreed with Light in the beginning, but he then began expanding his list of victims until it would have became anyone he didn't deem suitable in his eyes. In killing all those criminals, many a time he might have been killing off innocent people, which he did anyways. All the while thinking that he was justified and correct for what he was doing. Like a true psychopath.
Lelouch, otoh, is very well aware of his actions and the consequences they bear. It's war and sacrifices must be made. Whether his reasoning, he's not a complete psychopath/delusional like Light. He still has some kind of humanity left in him.