And he's in control of his emotions enough that merely being called a coward wouldn't set him off that way, either.

Whereas being called a coward after you've screwed up your courage to kill the only person who ever really believed in your loyalty, for reasons you can't explain in the middle of a crowd of Death Eaters, might sting a little . . .

(As you can see from this, I am holding out for the hope that Rowling hasn't moralistically shunted Snape off to the Death Eaters without a backward look: that would disappoint me extremely. Of course, now we have to wait two years to find out for certain . . .)