Towards the end of the book (Chapter 27-The Lightning Struck Tower), at the climax of the story, after Snape has killed Dumbledore and Harry goes chasing after him, there is an instant where Harry can't use cast an Avada Kedavra curse because he's angry but in spite of it, he doesn't have the 'desire' to do it. In any event, he calls Snape a coward. Which elicits from Snape a very angry reaction.

Obviously you could argue that Snape was just angry because he had to kill someone he truly looked up to, someone he respected and loved excuse me while I gag. But for those of us who know the truth some of us would disagree. Was there another reason maybe he reacted so wildly to that particular insult? Is Snape a coward, or was there something he did in the past that might be seen as cowardly? If so what do you think it was?

Is Snape a coward or was he just lashing out because he was angry at having to kill Dumbledore?