Had Snape not killed Dumbledore and remained as a double agent until Dumbledore died from the ring (or drinking the poison to get the locket) it is possible that he would not have gained as much favor from Voldemort.

When Voldemort succeeded in taking over the ministry, with out as much favor Snape may not have been named Head Master. Had it been one of the Carrows instead, Neville, Ginny and the rest of the DA would have had a much more difficult year at school (and may well have ended up dead). Snape would not have been able to get the sword to Harry, meaning all the horacruxes were not destroyed and Voldemort would not be defeated.

Allison//Ravenclaw