Aye...
Being screwed always hurts the most because 1 - it's not your option and you don't control it and 2 - it often comes as a complete surprise and utter shock, it's not something you can recover from very easily.
However, the weight on someone's conscience when screwing someone over, and is aware of it (many people convince themselves they're oh so wholesome and just), is sometimes borderline unbearable.
Still, it's something you can control and you can prepare yourself psycological to do it, and think of the words you're going to say when do the "terrible deed". This is in the cases when you screw someone over but you don't want to deliberately hurt them a lot... if one's intent is to hurt the other party with extreme prejudice, one is, by definition, guilt free and doesn't give a crap... thus, no regrets or heavy conscience...
Human relations... complicated matter as always...