Well, I think that
Snape's love for Lily was legitimate. We even saw that in OoTP.
I guess
Snape never really gave Lily that chance to love him back. Had he not made friends with others who fancied dark arts too, he probably would have gained that love from Lily too.......
Lily was a good friend of her, then he lost her due to his own mistakes.....
Still Love is love....... even one sided, he knew that he loved Lily, and he was the major reason she ended up dead. He spilled all the beans in front of
Voldemort about the prophecy.
Now he knew he owed her, and after DD accepting him to the light side, he owed even bigger favor to
Harry and DD. He ended up fulfilling it....... gave his own life to save
Harry.
And to answer your confusion towards why other teachers like Minerva, Sprout, and Flitwik didnt rebel against
Snape........ you are forgetting one thing. When
Snape became the headmaster, Ministry was lost to
Voldemort. There was not chance of fighting alone like that. Those good teachers stayed at Hogwarts to protect children from death eaters like Electo and Amycus, and of course
Snape.
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