Yes, evolution claims to explain our moral feelings, but if Darwinian evolution is true, then in actuality there is no such thing as morality. There is no set of moral do's and don'ts. That is my point and that is what Hitler believed.
That is not the right syllogism.
P1 Human beings have feelings about morality that evolution claims have an adaptive advantage for us
P2 But no one really knows what adaptive advantage means.
If you mean, "nobody can define 'adaptive advantage'," then the premise is false. We can easily define adaptive advantage as "something which makes it likelier that its possessor survives and has offspring." If you mean "nobody can determine in advance, with certainty, whether or not a particular action will help them survive and have offspring," that's generally true but it's irrelevant to your conclusion.
C Therefore, if Darwinian evolution is true, there is no such thing as morality, therefore you can do whatever you want if you think it is an adaptive advantage for you.
There is no "therefore" here at all. This is not in any way a valid proof, because the conclusion is a complete
non sequitur. Your two premises don't add up to any conclusion at all, so far as I can tell.
"If Darwin was right about humans having moral feelings because such feelings are an adaptive advantage, there is no such thing as morality" makes no more sense than "If Darwin was right about humans having depth perception because such perception is an adaptive advantage, there is no such thing as distance." In both cases it is entirely possible that evolution provides us with the mental or visual tools to perceive
something that really exists. (In the latter case, obviously, it's certain that this is what has happened.)
Not believing in an objectively real morality WAS relevant in the development of Hitler and the Nazis.
In Mein Kampf Hitler argues that all ethical and aesthetic ideas - indeed all ideas except those that are purely logical deductions - are dependent on the human mind and have no existence apart from humans, who have not always existed.
If you're talking about the passage beginning "Certain ideas are even confined to certain people," this is not an argument that morality has no real existence, it's an argument that certain political ideals -- the example he gives is pacifism -- are only "carried" by certain races, presumably because they are the only ones with the mental or spiritual capacity to conceive them. And of course at no point does he say or imply anything like "As Darwin proved..."
So, you still have not offered any reason to believe that anything Darwin said about evolution and natural selection could in any halfway rational way be "translated" into Nazism. If they could be so translated, but only in an entirely perverse way which employed wild leaps of illogic ("Darwin said that all animals compete, so it is right for me to kill my enemies"), that is not the fault of Darwin or Darwin's writing.