What do u call the idea that one person taking more than his fair share of something can ruin the system for everyone

A person who don’t care about no one else’s feelings regarding his own

I’m not sure if this but when beowulf enters, he told hrothgar that he heard about grendel's attacks and herot is now empty because of the beast that comes at night. beowulf also boasted about his abilities and his achievements. hrothgar told beowulf that his father sought sanctuary after ecgtheow killed heathloaf, so hrothgar made a treaty with them by giving them treasure. i’ll see if i can find anything else. i’m assuming he promises to get rid of grendel and when he fights grendel he takes off his arm and hangs it up as a prize.