Here's my thesis, in the spirit of creators trying to send a message and missing what they actually communicated (Zootopia, Bugs Life, Watchmen, etc)

Snape should always have been black. His entire arc is being unaccountably nasty to splebody because of the actions of their ancestors that they had nothing to do with.

He is protected and enabled by the institution out of misguided pity and the belief that he HAS to be protected for the greater good, despite his objectionable nature. The Goods Guys MUST put up with his antisocial behaviors. And while he is opening curel and injust they are required to treat him as a legitimate authority figure.

His backstory even more so, he is driven primarily by his lust for a redehead. He fails to won her affections by being a horrible person, and becomes rageful and entitled when she choses someone else. He lashes out both at her and her legitimate romantic partner. Attempting to downplay Potter's achievements in comparision to his own non-achievements.

If nothing else, disgusting black lusting possesively over pure redhead is probably not the image they were going for but its absolutely the one they've creating