Voldemort was very cruel to Snape, unnecessarily so.

Snape delivered a prophecy that Voldemort thought was life-saving, and he didn't even need to do it. He voluntarily did it! He wasn't tasked or anything. The least Voldemort could have done was to try to spare Lily, who was a Muggleborn, yes, but also very young and not particularly important. It would have cost him nothing to spare her, and he still killed her! He didn't keep his word. He could have stunned her but his promise to Snape (who had saved his life, remember?) mattered so little to him he still killed her for no reason at all.

Promising to spare Lily was never a big deal by itself because it would have been extremely demotivating and humiliating for a Death Eater to deliver something so significant involuntarily and for the Dark Lord to not even agree to a simple request. Voldemort always talks about how he rewards his followers but he humoured Snape, nothing more. The fact that he agreed to spare Lily for about five seconds before killing her anyway just shows how meaningless Snape's plea was to him.

Snape's one request, the smallest, most inconsequential request imaginable, was ignored. Voldemort couldn't even keep a simple promise that would have cost him nothing, and to someone like Snape, who was already rejected and humiliated by everyone and found belonging among the Death Eaters, that moment would have been shattering. It would have made him realise he didn't belong there either. He must have realised, right then, that he could pour his soul into serving Voldemort, risk everything, and still be discarded like rubbish, and he was right.

my Lord, their resistance is crumbling -'

'- and it is doing so without your help, said Voldemort, in his high, clear voice. 'Skilled wizard though you are, Severus, I do not think you will make much difference now. We are almost there... almost.'

It's Snape's life he's talking about here! For someone so terrified of death himself, it's almost as if he is taunting Snape. Voldemort, of all people, should have understood why Snape does not want to be killed lol. What's even crueller is the fact that he used Nagini to kill him. It's true that he didn't use the Killing Curse because he knew the Elder Wand wouldn't kill its true master, but he could have at least tried to make it painless by using another wand. Instead, Voldemort had Snape mauled to death by a giant snake just because he couldn't bother to bring his old wand with him!

He did not have to kill Snape in the first place and he knew that. He was aware murder wasn't necessary to win the Elder Wand's allegiance because he knew Dumbledore won it without ever killing Grindelwald and he had two expert wandmakers as his prisoners who would have confirmed this, but he still killed Snape on a whim.

'I regret it,' said Voldemort coldly.

He turned away; there was no sadness in him, no remorse.

He's really twisting the knife here with the cold 'I regret it', followed immediately by his own declaration of his true feelings in his own mind: there was no sadness in him, no remorse.

He rescued Bellatrix from the Ministry and squandered two years of careful planning to remain secret. There was no reason to save her from temporary imprisonment because he didn't need her for anything until he'd freed everyone else who was captured. Not to mention that the Dementors were under his control at that time and he deliberately delayed rescuing everyone else for a year, probably just to punish them. What made Bellatrix so special that Voldemort didn't lift a finger to grant Snape's request for sparing Lily, a tiny, harmless mercy after he delivered a lifesaving prophecy, but destroyed two years of planning to rescue one woman?

He did this even though Snape was more useful than Bellatrix, something he constantly taunted her with in Spinner's End. Bellatrix failed a lot and her usefulness was limited to being a duellist, which eventually served her more than it served Voldemort because she enjoyed it tremendously and the battlefield was her element, whilst Voldemort had armies of inferi, giants, other people, and was incredibly powerful himself. He also taught her the Dark Arts himself, so it was all much more beneficial and useful to her than to him, really.

On the other hand, in Voldemort's eyes, Snape spent decades collecting information and proving more useful than every other Death Eater and never failing, but he still killed him for the Elder Wand despite knowing murder wasn't necessary to win its allegiance. He killed Snape on a whim and did not even try to spare him, and why?

it seemed more prudent to finish them all.

Full circle.

Voldemort never fully trusted Snape and that's a fact. There are so many signs. Yes, he was arrogant and underestimated Snape's talents because he couldn't accept that anyone could ever be a good enough Occlumens to evade his Legilimency, but he still had doubts and got rid of Snape at the first opportunity despite knowing there were other ways to win the Elder Wand.

He was constantly using Legilimency on him even after he killed Dumbledore and Snape himself said Voldemort only believed him because he read his mind. Bellatrix in Spinner's End had information that wasn't public knowledge and only Voldemort, Snape, and Harry knew it (Snape preventing him from procuring the Philosopher's Stone) as LV didn't mention it in the graveyard, which clearly indicates they had a discussion about him where he shared his mistrust of him with her but they both reached different conclusions. Since Bellatrix escaped Azkaban six months after Snape rejoined Voldemort, it's obvious he was still not sure of his loyalty.

Voldemort sent Wormtail to Snape's house and he was constantly spying on him, most likely to report back to Voldemort. Voldemort knew exactly what he was doing by making Snape house Lily's betrayer. Poor Snape couldn't even take his grief and anger out on Wormtail because that would give away his love for Lily!

Snape spent his entire life being used as a pawn by people who did not even try to hide their dislike of him. Yes, he was working against Voldemort the whole time, but Voldemort didn't know that and he was still extremely cruel to him. Dumbledore wasn't much better to him either and even Harry only started respecting him posthumously, despite knowing since his first year at Hogwarts that Snape tried to save his life.

I feel so bad for him.