U.S. Federal law permits you to defend yourself if you have reason to believe that your life is threatened. The amount of retaliation tolerated by law depends on the predicament.
Any force used after the threat of danger is gone is considered excessive.
Even if an intruder is on your property, unless your personal safety is threatened, you may be criminally liable if you cause him or her harm. Consult an attorney in your area for more information regarding self defense.