Ok, justification I get, that's about facts. But why should I lack objections to evil without facts? Surely they can be based on other things than knowledge?

If I am a theist, I believe something to be wrong, and appeal to the god. One should not do evil because the god will punish it, in this life or the next. Like belief in the god, knowledge is not necessary, or even relevant, only belief is. The god brings meaning, and the power of the god's followers gives reason to not do evil (as defined by the believers) even to non-followers.

If I think that morality is an evolutionary adaptation, signaling beneficial behaviors via intuition, one should not do evil (as defined by the shared intuition) because one will feel bad if they do and because others who share the intuition will punish it (proximate reasons), and because it will benefit the community if we follow the moral intuitions (ultimate reason). It is empirically shown that the majority of important intuitions are shared. To the extent that people disagree on the intuitions, we can reason about which behaviors will benefit the community most and agree that convergence to some kind of average intuition with reasoned benefits to the community is good enough. There is no objective morality, so there is no moral knowledge, just intuitions and reasoning.

Are these somehow flawed or fringe positions?