It doesn't need to be factual.. Humans are highly intelligent creatures and also highly emotional. It is part of our nature to hold on to things longer than animals, mourn longer, and have an understanding of right versus wrong. It's not just a social construct- like table manners or opening doors for women- it's part of our evolution and brain size, being able to realize that it is wrong to inflict unnecessary pain on others that we ourselves would not want. It's about being intelligent enough to identify very basic morals. I mean, on an instinctual level, most people would be appalled and frightened to witness torture, rape, murder, etc because it's inflicting senseless pain and death on another. The sight of blood and gore causes a natural response of adrenaline- fight or flight. It is absolutely unnatural to relish, enjoy, or even be okay with these things, despite that you say it isn't a "fact". If you yourself do not want to experience rape, torture, and violent slaughter, (and 99.9% of people do not, by the way) then logically you would not inflict it on any other being- because it is, in fact, cruel.

Just because we have large brains that are capable of understanding concepts bigger than any animal can, does not mean our concepts are invalid or false. It's part of who we are, where our evolution has led us. Of course, morals can differ from person to person (i.e. One person finds gay marriage a disgusting crime and another does not) but I'm just saying there's absolutely a reason why almost everyone is opposed to torture and murder.