The beheaders of Nick Berg also undoubtedly thought he was subhuman, or they couldn't have done what they did. But this is where it always leads to think of other humans as subhuman.
I see what you are saying, but I would think that for normal people who have the basic sense of being humans, to have a natural basic compassion for others...seeing others commit these acts leaves you with such an emotional impact that the only thing you can come up with to explain why they did this was that that have no normal human feeling.
The difference between what the killers view of Nick Berg, and our horror at seeing what they did to him and calling them sub-human is that Nick Berg did NOTHING wrong. It's like having your daughter kidnapped and murdered. The murderor is in the wrong. The cold-blooded murder of a defenseless person is wrong, has no basic humanity. Yes, it is wrong to call or view people selectively as a race as subhuman and do some type of ethnic cleansing. Hitler was subhuman for doing that, as far as I'm concerned.
But somehow we have to make sense out of something like a child's being murder or an innocent man being murdered...because it defies humanity.