Voldemort told Snape he had been a faithful servant, but that he had to regrettably kill him, as he mistakenly believed he was the master of the Elder Wand, as Snape had killed Albus Dumbledore, its former owner the year prior, during the Battle of the Astronomy Tower.
"I regret it," said Voldemort coldly. When Voldemort finally works out the truth about the Elder Wand (albeit incorrectly) he sends Lucius to fetch Snape and then confronts him before killing him off. He confesses his apparent “regret” for having to kill him, twice.
Voldemort trusts Snape because he's good at his job as a double spy and as an Occlumens. Voldemort is so conceited that he probably just doesn't expect Snape could be a better Occlumens than he is a Legilimens.
Voldemort did get to know about Snape's betrayal. Harry himself tells him right before their final showdown. This is their final dialogue: "I brought about the death of Albus Dumbledore!"
He doesn't trust Snape as Dumbledore does, but he believes that he can detect any disloyalty. Moreover, Voldemort regards Snape as highly intelligent and therefore ' as a true Slytherin ' a person whose top priority is to take care of himself. To his twisted mind, selfless equals stupid.
I cannot deny, however, the fact that Severus was loyal to Dumbledore, to some extent. He responded very differently to Dumbledore than he did to others. He wasn't rude, sarcastic, nor bitingly cruel to Dumbledore as he seemed to be to many others.
Voldemort thought Snape was the owner of the Elder Wand. Using Avada Kedavra while the Elder Wand was resisting Voldemort was already a risky business, but using it against the true owner would be suicide (as Voldemort would find out when he used the Elder Wand against its true owner: Harry).
When Snape heard the prophecy and pleaded with Voldemort to spare Lily's life, he may have inadvertently caused his downfall. While we know Voldemort killed Lily, he did give her a choice. If she stepped aside and allowed him to murder Harry, he would spare her. Lily refused and sacrificed herself to save her son.
Now, as firmly established, Snape was not the greatest fan of Harry, but that didn't mean that he ever stopped loving Lily. Dumbledore was surprised that Snape seemed to care for the boy. With a swish of his wand, Snape conjured up a Patronus – Lily's Patronus, a doe. 'Always,' he said.
In order to preserve his position as a trusted Death Eater, Severus Snape was forced to tell Voldemort the true date at which the Order planned to move Harry, under the orders of Albus Dumbledore's portrait.
Erroneously believing Snape is the master of the Elder Wand and that Snape's death will make him the master of the Wand, Voldemort kills Snape by having his pet snake Nagini bite him through the neck.
He would always resent Harry for being James' son instead of his. Snape even said that he didn't want anyone to find out that he was protecting Lily's child, “especially Potter's son” (pg. 679 of Harry Potter and the Deathly Hallows) because of his hatred for James.
In the final book, “Harry Potter and the Deathly Hallows,” Rowling reveals that Snape was acting on Dumbledore's orders and had protected Harry at school out of love for Harry's mother, Lily. “Snape died for Harry out of love for Lily,” Rowling said.
In this story, Rowling introduces him as the Dark Lord who tried to kill Harry Potter because the boy was prophesied to destroy him.
He truly believed the Elder Wand had to be passed down through murder, because that is what the legends say. But even if he knew that one only had to be disarmed, Voldemort was still too arrogant to do that. He kills anyone in his way.
As Rowling explained in a series of tweets, Snape loved Harry's mother, Lily, and his unrequited affection drove him to resent Harry's father, James — and by extension, Harry himself.
In "Deathly Hallows," Snape also uses his Patronus to help Harry on his quest to find Voldemort's horcruxes; a Patronus charm can be used to send messages, and Harry seems to subconsciously recognize the doe as an extension of his mother, allowing him to trust it.
They were close friends, she saw in him someone special that she could care about. But she couldn't be with him. So, it seems like it was very unlikely that Lily ever found out Snape loved her.
Snape calls Lily a Mudblood, Lily tells James that she would never date him because he's an egotistical jerk. Later, Snape tries to apologize for calling Lily a Mudblood, but she's not interested; she ends their friendship due to his association with Death Eaters.
voldemort was trying to spare her, but he didn't care that much if he succeeded. this is basically what happens in rowling's canon.
Snape apologized because Lily ended their friendship over it. Lily was done being his friend. There was no reason for her to apologize.
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.
During that time, he subjected thousands to death, torture, and despair. Nobody but Harry is able to defeat him, though, meaning he is never incarcerated. Voldemort would have probably had the magical prowess to escape anyway, particularly if Sirius Black can do it.