i agree with ginnyweasleyrocks
maybe there was a lot of hatred in snape's face because he did not want 2 hurt dumbledore and i guess he has used some jinx in the non verbal spellss
also when harry was caught of using the rectumsempra spell by snape, snape knew that harry had his book becuse he was an accomplished legilimens and could read harry's mind also that spell was very bad and was carried out on his favorite student draco malfoy yet he gave harry only a couple of detentions.