If someone cares more about themselves than continuing their family line or the human species, it’s arguably more selfish.

People are a more intelligent type of animal but still animals.

Sure, it's a pretty high chance they will go into space. If they use energy storage it could even be trillions of years. It really depends if humanity self-destructs or not.

Also different animals kill each other all the time. It's part of the cycle of life. I don't get why you have such a negative perspective.

If you have children you can help continue a chain of humans millions or maybe even billions of years long.