Excuses to be nasty and do the wrong thing?

I haven't really lost anyone close to me (imediate family and close friends) so I have no idea how it feels to experience such pain... But should losing a loved one be used as an excuse to be nasty to people who's trying to be there for you? Should it be use as an excuse to cheat on your SO and push them away regardless of how hard and sicerly they're trying to be there for you?