How should a teaching-assistant deal with an angry student?
As a teaching assistant, you should not be abused.  If a student becomes angry and abusive, you need to disengage and pass the problem upwards.  While TAs participate in the grading of a course, the responsibility and authority for dealing with any significant grading dispute with a student lies with the professor, not with any TA.  It's OK for TAs to deal with minor problems presented calmly (e.g., "You added up the scores and got the wrong total"), but major disputes and angry students are not things a TA can or should attempt to handle.

If you are confronted with an angry student, do not respond to the content of the dispute.  If you do so, you may say something that can cause a major problem for yourself or the professor later.  Instead, just keep repeating something along the lines of,


  "Please calm down. This is something that you need to talk to the professor about."


If the student persists to the point where you feel threatened, then you have both the right and the obligation to get campus authorities involved.  A person who is extensively verbally abusive may become physically violent as well, and you need to protect yourself.

Once you have disengaged from the student, contact the professor immediately and explain the situation.  At that point, it is the professor's responsibility to deal with the student, though they may need more information and input from you in deciding how to proceed.