If you have a negative utilitarian moral outlook, it follows that if some beings will suffer by existing and its not possible to know which beings they are in advance, then bringing any beings into existence willfully is morally wrong. I am not a negative utilitarian myself, but the position is not selfish, crazy, or based on personal unhappiness: people who themselves are very happy can be negative utilitarians. If those happy negative utilitarians see unhappy people in the world, they can then come to the conclusion that they should be antinatalists, even if they are themselves happy.

By the way, asking people "why don't you kill yourself" is not very nice. Even if you disagree with someone's moral philosophy, jumping to "why don't you kill yourself" isn't going to win anybody over or garner you any understanding.

https://en.wikipedia.org/wiki/Negative_utilitarianism