How can someone who believes the prime motivation for human existence is Darwinian not become a social Darwinian
Can you point to anybody who actually believes this? No, you can't, because you're committing the howling category error of mistaking a fact (
human existence comes about through a biochemical process that some rather behind-the-times people call Darwinian) for a belief about the purpose of individual lives. The
is of human biological evolution does not lead to any
ought. Which is not to say that beliefs about the latter are not held, because they are, but on the whole anybody who ties a belief system to evolutionary biology tends not to understand evolution very well (or at all); rather that such stances are chosen for quite different reasons, not because there's a line from evolution to them. There isn't. You can't get there from here.
or a materialist avoid the descent into acquisitive materialism?
Aaaaaaaaaaand there we go again - conflating a philosophical view (
matter-energy is all there is) with a behaviour built upon entirely different foundations.
Speaking of acquisitive materialism, have you ever seen the Vatican?
0/10.