Yeah he could've. But I think at a time like that, you rarely think clearly. You pretty much use the first spell that comes to mind. I would also point out that, in the amount of time it would take harry to say the entire spell, the death eater would've been able to say protego quite quickly, since it's so much shorter. Harry really shouldn't have been a slacker, and practiced silent spellcasting more studiously. That sometimes drives me nuts about harry. I mean he knows clearly that EVERYTHING he learns in class is of vital importance in the real world. Why the HELL doesn't he study harder, and forget about the retarded stuff like Quidditch? I mean really, if he learned silence spell casting better, he might have been able to at least maim snape with a kiling curse, seeing how he probably doesn't have enough hatred to kill him.