Atheism, A Grim Position?

[JuliaL]

So then if "Maximizing my lifetime total of pleasure" entailed killing redheads because it was pleasurable, does that go against atheist ethics? If so, how?

For every complex question there is always a simple wrong answer.
There is almost nothing simpler than 'God said to' so theists go for that.

If there is an absolute, God given, proscription against killing people, why is it that the maker of that law allows, no, insists that mama bears protecting their young kill hikers? Is it moral? immoral? amoral?

Morality is a set of constrained behaviors specific to a replicating pattern which improves its chances of persistence and replication. In the case of 'thou shalt not kill' the replicating pattern is that of social human interaction. Given two societies; in one of which everybody always choses to kill whomever they saw, family or not, friend or foe. In the other there was a rule, written on papyrus, engraved in the hearts (or neural nets) of its members which said, 'lighten up, don't off Fred (or redheads.)' even when you really wanted to. Which of these societies would better survive? The privilege to travel into the future is the only valuable commodity we have. Reality has shaped our minds and bodies and societies to do that via descent with modification sieved by natural selection.
The societies we see and the empathy we experience are the result of this process. These are the behaviors which have survived and propagated. It isn't magic. It isn't absolute. The thou shalt not kill rule pretty much only prescribes against human vs. human killing which is exactly how you would expect it to read given that it essentially only applies to societies protecting their components against each other. Of course, when you look at that don't kill rule from the standpoint of a couple of societies living next to each other, it gets modified to 'don't kill members of your group.' The other guys? Go for it!
The bear is outside those human systems and not held back. She is probably thinking: love cub, kill threat. Or maybe she just likes the way hikers taste.