We know from canon that to become a healer, potion master or auror, you need to have potion NEWTs. If we combine that with Snapes horrible teaching as well as his requirements of an O to be able to go to NEWT classes, we can easily assume that he solely was responsible for the decline of all three professions. And two of them were actually important for the war, so due to the shit he was pulling he helped Voldemort immensely to help win the war.
If it weren't for the fluke at the end with Harry owning the Elder Wand, Voldemort would never have lost the grip over magical Great Britain to begin with. Additionally he helped to make the other three houses hate the Slytherins and due to that a lot of Slytherins actually joined Voldemort cause. They pupils were so bitter from the hatred that they potentially wanted revenge or the recognition with the help of Riddle.
Last he was the WORST spy of the whole world. Whatever he did for the Order of the Phoenix, was worth shit. He didn't help them at all, except some talking to Dumbledore, who did not disclose any information to the members anyway. Furthermore he gave Voldemort ALL the information he needed about the order. Remember Harry escaping Voldemort during 7th year with his clones? Was only possible because Snape ratted them out and of the best duelers in form of Alastor Moody died there as well. He has so many deaths on his conscious...
Thus in my opinion he is irredeemable...