Is it understandable that Snape feels the way he feels? Absolutely. Is it acceptable that his actions in response to his own feelings continually harm others? Not so much.
The point is not that Severus Snape was a monster and no one should ever think well of him. The point is that Severus Snape is not a hero, and wouldn’t want to be called one. He is a man burdened by real demons, who makes the wrong choices, who pays for it with everything that is dear to him. And he’s the one who makes that bed. He knows he has to lie in it, knows that’s what he earned for himself, and that’s why he does everything in his power to make it right.
It’s what makes Severus Snape such a mesmerizing character in the first place. He doesn’t want to be coddled by anyone who feels for him, who wants to ease his pain. He would probably feel pretty awkward about Harry using his name to christen one of the Potter brood. Severus Snape doesn’t need pity because he’s not meant to be pitied—the owning of his failures are what make him exceptional.
And that is far more interesting than being a martyr any day."
I felt this was relevant due to the drastic vilification, and martyrization on the other side, of Snape that has been occurring recently.
So apparently there’s only a few steps between, “Hey, I’ll give NaNoWriMo a shot!” to, “I’m staying up late to invent languages for the fictional countries in the fantasy series I’ve been working on since 2005.”