OMG How did I never know about Snape: A Definitive Reading
The introduction of that book is just mesmerizing. The way Lorrie Kim describes Snape.
Since you have written a lot of Snapefics, I am curious to know what you think of A Difference in the Family: The Snape Chronicles By Rannaro
I don't think that's how Rowling sees it or meant for the character to be constructed. The theme that runs through the books is the inner magic that manifests in love- love which gives of oneself without the expectation of receiving something back. There is a possessive disposition which piggybacks on love (as all evil must piggyback on something good- even those who torture children do it for pleasure, that being an innate good, rather than for its own sake) and twists its character, and while that disposition is present in Snape, especially in the memories which we see in the Pensieve, during our lives such twisted manifestations of the good are rarely undiluted in their evil- they have a trace of the reality in them, a trace which can be grown and fanned into flame.
Lily was dead. This wasn't a possessive love which expected that she would one day emerge from the grave and sleep with him. The reason that Voldermort never suspected that Lily's death had any real impact on Snape (we see this come out during his final conversation with Harry before being killed) was that genuine love was so utterly inconceivable to him. What he doesn't understand, he doesn't take into account. Snape did what he did because that love which had become twisted and possessive had a trace of the real thing in it: to value a person for what makes them who they are, for the good which shines out of them. And so for that reason, he risks his life daily to protect a boy whom he despises, with the knowledge that he will never be paid back or recognized, that he will be thought a dishonorable traitor, and that he himself may die a violent and horrific death.
We should also be asking the question of why Snape selects the memories he does for Harry to see. He grew- especially in that final year- throughout the stories. Why did he choose to show Harry the moment when Dumbledore told him he was wrong in thinking Harry just as much of an arrogant prig as his father (I'm not one of those who hates on James, FYI, just speaking from Snape's perspective)? Because he recognized during that final year that Dumbledore was right. Those same qualities which he valued in Lily did shine through in Harry.
Again, I'm not saying Snape is "good." I'm saying that to read him as a horrible person who did the right thing for not-right reasons is too simple. Love- at least in the mind of the author, and I know some people are critical of putting this weight on authorial intent- is the right reason which makes up the rightness in everything else. In any case, if you're interested in a different perspective the following book is a terrific work of literary analysis and has very much shaped my reading of HP:
https://www.amazon.com/dp/B01I04EMK6/ref=dp-kindle-redirect?\_encoding=UTF8&btkr=1
Your best bet might depend on what part of the timeline you are writing, and how you need to address his insecurities for that part of his life. Is this a Marauder-era Snape dealing with his path toward becoming a Death Eater? Series years trying to balance his double-agent roles? Post-war romance/mystery/comedy? Decades change a person, and Snape has been through some serious shit, so think about that for sure.
Ever since I read it, I've been recommending all writers who want to get into Snape's head should find a library copy or cough up the 5 bucks to get Snape: A Definitive Reading. Many of my own fics are post-war EWE but otherwise canon-compliant and this analysis has helped me significantly to keep him in character.
That said, he's the mysterious magical spy, so if you can find any shred of canon evidence for the behavior you want to give him, or even a halfway decent explanation for why the canon behavior is not his true self, then you can probably make it work!
Also, all of my fics are Snapefics if you care to take a look.