The fact that Marico was innocent didn't negate the fact that he had indeed murdered several people even if they weren't exactly innocent. That kind of guilt could be amplified by solitude and break a man's will, no matter how strong.
Zach grew worried about Luke because he knew that the prince had a strong sense of justice. Such people were the most adversely affected by moral dilemmas.
For example, choosing between an old man and a kid wasn't too difficult for a villain with no value for human life. He could instantly make the decision to kill one of them to save the other, or he could kill both of them because it didn't matter to him either way.
But a righteous man who believed in justice would study the situation from every perspective and would end up lost in his struggle to do right by both the old man and the kid. He would try to find a righteous reason for his action in a situation where there was none.
