Establish boundaries. Knowing each other's limitations early in the school year can help keep both of you from getting on each other's nerves. Discuss pet peeves that each of you has, such as music played too loud, friends who visit every night of the week or dirty laundry left on the floor. Knowing what annoys your roommate will help you avoid unintentionally starting a fight.
Be nice. Responding to your roommate's mean behavior with an equally unpleasant one will only exacerbate the situation and cause more feelings to get hurt. Instead, if you sense that your roommate is angry about something or just in a bad mood, remain friendly and approachable and see if a positive approach will help smooth the situation over.
Talk it out. If your roommate is reacting to your presence with mean or unfriendly behavior and you can't think of anything you've done to cause it, ask her what is wrong. If she is that upset with you, she should at least be willing to discuss the problem or admit that she doesn't have a problem with you. Explain that your feelings have been hurt by the way she has been treating you and that you want to know why she is acting this way toward you to keep the problem from recurring.
Stand up for yourself. No matter how different your personalities are, you still have a right to be treated with decency and respect by your roommate. You don't have to be best friends, but you should also not dread going to your room because your roommate is just too mean. If attempts to be nice don't work, be firm. Point out that you have tried repeatedly to fix the problem or talk things out and that you don't feel you deserve to be treated in the manner that he is treating you.
Report threats immediately. If your roommate's behavior escalates from being mean to being threatening or violent, make a report immediately to your resident adviser or campus security. If you feel that your roommate has the potential to do harm or turn violent, the situation must be taken seriously and addressed with the help of outside authorities to avoid any and all risk of danger or harm to you.
Ask your resident adviser to help. RAs often set up a sort of counseling session for roommates who simply cannot get along and feel that they are being treated unfairly or being disrespected. It will be an opportunity for both you and your roommate to voice your grievances about one another and hopefully come to some sort of resolution to the situation.