You are the one going ad hominem by attempting to tie the racist tag to Darwin.
But, if we are going to get into the inherent personalities of the individuals, the choice between an unbeliever who may have considerd Africans less advanced than Englishmen, but was nevertheless appalled by the idea of enslaving them, and a Christian who thought them created by God as draft animals, is a no-brainer.
I don't know enough about Agassiz to even know if he was actually a Christian (there's a huge gap between all those who claim to be, and those who actually are Christian), but your reference to him is still irrelevant, because he has no following nor influence.
Certainly, you can point to racist Christians, but as I stated previously, they don't have followers a hundred years later.
I'm not saying that anyone who believes in the deductions based on the guesswork of Darwin is a racist, as he was, but my point is that his devotees have a religious fervor that even the right wing fundamentalists can scarcely match.
It's a faith, and he's the founder. You may not be one of the faithful (I don't know that), but there IS a faithful, and they will fight to the finish for their beliefs.