In most of the roleplays I GM or participate in, the killing usually happens between monsters and humans. as a result there's a disconnect there. The predator does not feel sympathy for their prey. It's an entirely different story when both creatures are human, or identical in nature.

I find it's kind of hard to regulate how "gritty" killing someone should feel. This is mostly because each player and their characters deals with it in a different way. I know one guy who gives all of his characters personalities, even the "nameless guard," to help guide how you should feel. There are others who make killings look as simple as cutting a loaf of bread. I try to give a little bit of insight into the kind of characters that are being killed. If people are being killed by the truck full, it's not very convincing to make the characters feel anything anyway. That changes if the character is particularly important or close to the characters. 

I'm not sure making death heavy makes things more realistic, but I do think it connects the reader to the world better in certain situations. It really depends on how dark you want things to be.

As for "sadistic acts of violence like torture or rape," it depends, but I feel it misses the mark most of the time. I find torture and rape are abused as a sort of shortcut to make things dark on the cheap. The problem is that torture and rape can only be committed by people with very specific mindsets. When people without these mindsets commit these kinds of crimes, it tends to make the world feel more contrived and artificial.