What IS good, and how do we determine it?

[Esquilax]

It's almost like we instinctively know it's wrong. But where does it come from? And if someone was brought up being taught that it's ok to kill people, and they do so, they would still be doing something immoral. Because murder is immoral regardless of how the person feels about it.

You do realize you've said two mutually exclusive things, within two sentences, don't you? You began with "we instinctively know morality," but then a moment later you're asking us to envision a person who was brought up being taught that murder is morally okay, and thus believes that. So which is it? Do we instinctively know what's moral, or do we have to be taught? Your god clearly thinks it's the latter: despite claims of morality being written into our beings fundamentally, he still had to literally come down and tell us what's up. He had to step in: from his point of view, morality apparently has to be taught.

For me though, the source of morality is easy, though actually executing on it is less so: we live in a reality, that obeys consistent physical laws. We are biological organisms that, in turn, obey those laws as a function of our biology; what it means to be a living entity is an objective fact, what harms us, what benefits us, what allows us to survive, are all discoverable and consistent amongst everyone. Given this, there are certain fundamentals of being human that can be known to be universal: pain means bad things, pleasure means good things, we all know what injury is, what death is, and so on. I doubt anyone would argue that being mauled by a cougar is a good thing for us, at least in a physical sense.

We evolved a certain way, and what you see as an instinctive understanding of morality is actually just the execution of that evolution; we can tell without thinking too hard what events would be bad for us, and we have a natural aversion to those things, which are both good survival mechanisms, because you can identify danger and want to stay away from it. When you see these things happening to others your empathy kicks in, and you begin thinking transpositionally; what's bad for you is bad for them, and you'd prefer that it not happen to anyone else either because you can empathize, which is a good trait to have for a species that survives via cooperation and so on. I don't think I need to tell you that our position as the dominant life form on the planet is down largely to our willingness to cooperate and share knowledge.

That's really all you need, to think through a moral system. Basics lead to more complex moral tenets, as you encounter more scenarios and compare them to this common set of objective facts about being alive, in conjunction with your empathy. Yes, it's a big topic that's highly contextual, but it's not something that's impossible to reason out on your own. You don't need some god handing down moral laws, because by and large you already know what's good for you, and can extrapolate from there.