So given that we've had the same amount of time to evolve

And as you say we naturally evolve towards promoting less suffering

Why are we more evolved? Why is stoning adulterers still prevalent? Burning witches? Genital mutilation? What is the purpose for these rules?

****ty example.

Let's say we had the option to kill a child to save a hundred people, is that better?

If we can't find a way to save all then it's the child. It's morally grey, because nothing in life is simple, but it still does more help than harm.

Well, depending on who the hundred people are.

Traditions, dictatorships, desperation, etc. Various reasons.

if I see a homeless person walking down the street who is clearly suffering, and he has no family or friends to speak of, is it not a good thing for me to kill him swiftly and painlessly as possible?

I would be ending a lot of suffering, and causing little if no new suffering. So is it the right thing to do?

so then there are other factors that influence morality beyond utilitarianism. So what exactly makes it the defining influence?

Nah, **** this idea that children are more valuable humans.

It'd most likely be a terrorist situation where we're given a choice. If it's naturally, then maybe that's different but you really should consider killing the kid. The argument is the kid doesn't get the chance to live while the people do. We don't know the age of the people and just because you're an adult doesn't mean you don't still have life to live. The kid won't know what he's missing and there's a high chance that he doesn't like his life when he finally gets to live it.

Then again, of course we need to consider the wants of those affected. Lets say majority of the 100 people would rather die to keep the kid alive. Then lets kill the 100 people. Vice versa as well.

Just because it's a social norm or it's a rule doesn't mean it's moral even if they use that word to describe it.

Postmodernism is a ****ty faux philosophy that nobody can even agree on as a thing.

Edit: And who ever said that we're evolving in every way? We're evolving in some ways, perhaps devolving in other ways. But realizing that we shouldn't enslave or dehumanize other people based on their race is definitely an evolution.

No, because you're assuming that he'll always be suffering. You're likely preventing any chance of him to feel something that isn't suffering. It's also a very very lazy way to end his suffering.

No, it's absolutely a faux philosophy. Meaning it's not really a philosophy it's a movement made up of several philosophies which is why nobody can agree on what postmodernism is which is actually the most postmodern thing ever.