I sometimes have a suspicion that Christian apologist overstate the evil consequences of following humanist reasoning. For example, to say that holding a relativistic moral outlook implies that Hitler's views must therefore have been ok since he really believed in them, seems a bit of a misrepresentation of moral relativists.
However, have a look at this. Peter Singer, a philosopher from one of the most respected universities in the world, is actually advocating infanticide. He is doing it in his own words. It is not just where his views lead to, these are his views.