They can be quite mean to each other. If you put an adult male into a cage with a female with babies he will kill all of the little babies so the female will go back into heat and have his children. Also, like other rodents, if the cage gets too crowded the big males will start killing some of the others to decrease population density.
True, when I was a kid we had a pet hamster kill another for no apparent (to us humans of course) reason. Evil little shits!
