This article contains major spoilers from Harry Potter and the Cursed Child, and is intended to be read by people who have seen the play or read the script book and are ready to talk about the events. Read on at your own risk!
Harry Potter and the Cursed Child dove deep into the Harry Potter series, not only by picking up the story 19 years later, but by using a time-turner device to explore alternate scenarios to some of the events in the first seven books. Albus and Scorpius' efforts to use the time-turner to save Cedric Diggory caused a ripple effect on the timeline of the Harry Potter universe, and among the major alterations made, leading to what was certainly the darkest timeline, Severus Snape survives the Battle of Hogwarts, and Harry Potter does not. This allows us to see what would have become of Snape, and whether or not he would've continued fighting.
One could argue that the purpose of Snape's return in the play was to create an opportunity to bring this beloved character back to life, if only temporarily, and maybe that did factor in. During my viewing of Cursed Child, Snape got a huge round of applause the moment people realized who was on stage. It was definitely a moment. But I'm more inclined to believe that J.K. Rowling and Jack Thorne considered Snape's potential in this shifting-timeline story, and saw an opportunity to reveal the true depth of his character.
In the event that everything went wrong, would Snape have walked away? At what point would he have given up the fight? Cursed Child manages to prove that Severus Snape never would've given up fighting, no matter how bad things got.
We knew from Deathly Hallows that everything Snape did, he did for Lily, right down to his dying breath, when he gave up his precious memories to a boy he could barely stand to look at. This was proven from the decision to turn away from Voldemort and help Dumbledore, to his choice to protect Harry after Lily died, and all the way to the end when Voldemort ordered Nagini to kill him. Cursed Child presents us with a scenario in which Snape survives, but Harry dies. Would Snape consider his mission a failure and abandon it in favor of a simpler life? Or would he continue the fight, despite all being lost?
In the second part of Harry Potter and the Cursed Child, Harry Potter is dead, and Voldemort has risen to power. Things are about as bad as they can be. Rather than quietly retiring or worse, finding himself a powerful role in Voldemort's new reality, we find Snape secretly working with Ron and Hermione to turn things around. He hasn't given up, even though most -- if not all -- of his motivation to fight should be gone. And he continues to risk his life and freedom to fight what's left of the war. He actively chooses to be part of the dim light of good that's still flickering in an otherwise dark Voldemort-and-Valor world. His only real allies are "Granger" and Ron, two people he doesn't seem to like very much, though I imagine he harbors some measure of respect for them.
Snape is not on autopilot, however. He's not a man simply carrying on with the plan, even though everything has gone down the toilet. That much is made clear when he speaks to Scoripus...
It's likely a mark of the depth of Snape's love for Lily that he continued to fight. But it also demonstrates a strength of character that I'm not sure I would've been able to credit him for prior to Cursed Child.
The play takes Snape's redemption a step further, not only by showing him still fighting, decades after Harry has died, but by putting him in a scenario in which he has to choose between his own life and the greater good. Snape knows he's dead in the real timeline. So he has essentially nothing to gain by helping Scorpius put things right, and yet, when the time comes, he takes out Umbridge (hardly a sacrifice, but he gets points for it anyway), and allows himself to be overcome by the dementors so that Scorpius can escape.
Admittedly, prior to Cursed Child, I always questioned how much credit Snape really deserved for turning on Voldemort when he did. Don't get me wrong, I love that Snape's true motivation was centered around love. Love is a theme that has always resonated beautifully in this series, and Snape's story is no exception. And this is not to diminish his actions in the later years of the war, by any means. But when looking at the big picture that is Severus Snape, I had a hard time getting past the fact that he was a Death Eater up until the woman he loved was in harm's way. It just seemed a bit self-serving to me that he was willing to provide Voldemort with that key bit of prophesy information, and then hit the panic button when he learned that it was Lily Potter in Voldemort's crosshairs. My feelings are even further mixed by the fact that he seemed fine taking a childhood grudge against James Potter out on Harry.
Snape is one of the most interesting characters in the Harry Potter series, but there's no denying that he's flawed. With that in mind, his redemption in the books, and again in this play, is made all the more powerful. He has a tougher road than most characters in this story, and maybe that makes his actions count for more.
In Deathly Hallows, Snape put his life on the line to protect Harry and help him defeat Voldemort, and he died for it. Cursed Child put Snape in a scenario where he really stands to gain nothing, and still does the right thing. It really does reveal the best in who he was as a man, and who he would have become, had things turned out differently. In fairness to the character, Snape really didn't need to prove himself or his loyalty again. However, if there was any question about his bravery and ability to act selflessly, Harry Potter and the Cursed Child puts the matter to rest.
A wise man once said...
If that's the case, then there's no denying, Severus Snape is a hero. But we kind of knew that already, didn't we?
Before we wrap this up, it also needs to be noted that Cursed Child adds something the original seven books couldn't, and that's allowing us to see how Snape would react to knowing Harry Potter named one of his children after him. Snape never really got to celebrate his part in Dumbledore's Army. He was never really part of the family, the way the others were. It's highly unlikely he ever hung around to have a butter beer with the Order, most people barely trusted him (and only because Dumbledore did), and he didn't survive to celebrate the victory of the Battle of Hogwarts. Cursed Child offers Snape (and us) a small bit of consolation in revealing the depth of Harry's gratitude to him through the knowledge the one of Harry's kids bears his name. And from Snape's reaction, that matters...
RIP Severus Snape.