Summary: Severus falls back into old habits. Somehow Harry is involved helping him out of them.
Idea 1: After discovering the Dursleys/Umbridge/someone else is/has been hurting Harry, Severus kills them.
Idea 2: Dumbledore lets Severus take the DADA post, but his fears of it bringing back out Severus' dark side are proved correct.
Idea 3: Voldemort discovers Severus is a traitor, but rather than instantly kill him, decides to try and turn his loyalty back.
Idea 4: Dark magic is an addictive substance. Severus has been using some sort of drug to control his cravings, but now there's a problem with the supply.
Or anything else you can think of!
Any category and point in canon goes. No Dumbledore bashing. No slash. No dark!Harry.
Bonus points for a story involving more than one of the ideas. Extra bonus points for one involving all four!