Why did Snape react so angrily to being called a 'Coward' ??

[rowena_grunnionffitch]

No: HPFGUIDX 133835

  IMO Snape reacts so furiously to being called 'coward' because he
secretly believes it's true and despises himself for it. But of course
it isn't.

  Which ever side SS proves to be on the one thing he is *not* is a
coward. Either way he is taking an enormous risk as a double agent.
As I have said before, for sheer cold courage Snape is in a class by
himself - possibly only matched by Dumbledore.