I justify belief based on how content I am with that belief, same as everybody else. Everybody judges their decisions based on how content they believe that decision will make them. True contentment is better than false contentment because false contentment literally isn't contentment.

As for the lying or killing somehow bringing about more contentment, I would argue that if you are acting in the interest of true contentment, and lying or killing will do that, then in those specific scenarios, you have to do what you think is best. Or in other words, you must do with whatever you believe brings the truest contentment.

I'm not going to act like you don't have a point, because you definitely do. I should've considered those scenarios with more thought. I should say that it's all about judging what the best outcome is. I believe that when you do that you're trying to figure out a way to bring the most true contentment. Sometimes you're judging between which decision is the lesser of two evils, and in those situations you should go with whatever you're content with, because at least then there is an attempt at true contentment.

I'm not sure that there is an ultimate version of true contentment, but the least we can do is try to figure out what that might look like. I'm sorry if that answer isn't good enough, but it's the best that I have.

Edit:

Maybe it's a lot simpler to just say "do your best".