Many authors write things they regret—books, sentences, misplaced words, whole bodies of work. Some authors even regret the success of their books. But I'd like to focus on characters and their own regrets, or lack of regrets.
Depicted in some beautiful fan-art above, is Severus Snape I imagine as an adult, as yet to "betray" Dumbledore and fully become the spy, the informant, the double-agent(?) that he needs to be in order for Voldemort to finally be defeated. Snape is the proof that not everyone gets to be Harry Potter. Someone, in any war or conflict, has to compromise themselves (this story seems to say) in order for good to prevail; perhaps even for there to be a hero at all; a hero who does not have to morally compromise themselves.
Because heroes who don't compromise are treasured, aren't they? From superheroes, to Buck Roger-type sci-fi epic heroes, to (sometimes) the soldier who comes home from war, the hero is someone who is morally impregnable. Upstanding. Righteous. Would never debase themselves. Compromise themselves. Put on the "bad," "ugly," or "hated" or "evil" masks. Someone we envision ourselves as. Even if tempted, even if challenged, in the end this hero never compromises their moral code.
We wouldn't love and adore and uphold our heroes nearly as much if they did.
That's why I'm triply glad J. K. Rowling put Snape in the story. He's about as ambiguous, in terms of good and evil, a character as it gets. And in a story that so depends upon "the good side" vs. "the evil side," it's a master stroke to so heavily involve a morally ambiguous character such as Snape in the plot.
Snape shows us regret. He is himself an apparition of regret. Of living with that regret. Of regret defining his character.
Whereas Harry, and other characters, may have small regrets (wishes that they had done something different, or acted sooner to save someone) and some other characters may even have monster regrets (like Voldemort regretting, perhaps, that he never managed to kill Harry), it is only Snape who bears the deepest, least resolvable, truest regret.
That regret is part of the emotional core of the work. To Snape's character. A sense that he both despises Harry and yet is, arguably, the most pivotal help Harry ever receives in defeating Voldemort. That Dumbledore himself convinced Snape to wear that evil mask, so to speak. To trick Voldemort. To actively undermine him. That one of the best things we can do as authors is not create characters who are obviously on one side or the other. Characters who are cursed, in a way, to bear our deepest, greatest regrets.
Because without the sting of regret, there is no healing the hurt.
Because without regret, just how believable are our characters? Our stories? About us?