It should be noted that despite his fear of Voldemort and that he was not a Death Eater, Quirrell was one of Voldemort's only followers who referred to him by name, the others being Bartemius Crouch Junior and Peter Pettigrew.
Professor Quirrell was not a Death Eater. He was someone who had the misfortune to discover Voldemort in the forests of Albania. Quirrell allowed himself to be convinced that might made right, and that he should assist Voldemort.
Quirrell is, in effect, turned into a temporary Horcrux by Voldemort. He is greatly depleted by the physical strain of fighting the far stronger, evil soul inside him.
Quirrell knew it was Severus who interfered with a counter-curse when he tried to throw Harry off the broomstick. And Voldemort was there under his turban so he probably witnessed it too.
Voldemort controlled Quirrell as to attack Harry. This activated Lily Potter's magic where Voldemort is not even allowed to touch Harry. And that's why Harry was able to burn Quirrell down.
Hagrid later becomes friends with Ron and Hermione as well. Later in the book, a hooded person (Quirinus Quirrell in disguise) gives him a dragon egg to elicit details about Fluffy.
He explains also that Quirrell could not touch Harry because Harry was protected by his mother's love.
Snape's fierce devotion to and love of his childhood friend Lily, Harry's mother, is the foundation of that loyalty. After Harry Potter and the Half-Blood Prince, Snape's loyalty was a matter of intense debate among the fans.
He didn't need to tell Dumbledore his suspicions because Dumbledore was already well aware: it was Dumbledore who asked him to keep an eye on Quirrell. It's explained in the last book.
In order to conjure the avada kedavra curse, you have to want to kill your victim. We all know that Voldemort could easily kill a child without an ounce of remorse... but not Snape. Snape didn't want to kill Dumbledore, and this was why the spell was blue instead of the usual green.
The villain in the "Fantastic Beasts" movies is Gellert Grindelwald, who played a small but crucial role in the "Harry Potter" series. He was the wizarding world's biggest villain before Voldemort came along. In his youth, Grindelwald was friends with Albus Dumbledore, who was in love with him.
Quirrell didn't use the Killing Curse on Harry, because Voldemort said “use the boy” to get the Sorcerer's stone from the Mirror of Erised. If Quirrell had killed Harry right away, he would have lost the chance to get the stone.
He was once again a disembodied soul, unable to use magic as he did not have any physical form, weak and helpless. He only managed to regain a weak human form as seen in the Riddle house when Wormtail helped him concoct a potion.
Quirrell is defeated by Harry upon physical touch, which burns him and allows Harry to break the chokehold. Harry cuts off his access to the stone and touches him, forcing him back, and he disintegrates, crumbling to ash and dust at Harry's feet, dead, and becoming Harry's first kill and victory.
" Quirrell tries to kill Harry for the stone, but Harry kills him by touching him, which blisters him and turns him to dust. It's possible that it's actually Voldemort's presence within Quirrell that's allowing him to perform such feats, but again, this is never explicitly stated. Voldemort werewolf.
This theory suggests that each time he created a new Horcrux, that specific Horcrux diminished a part of his soul. Thus, his nose began to sink in, eventually disappearing altogether. Due to his tampering with dark magic, his human appearance evolved into a more snake-like state.
The truth came out when Professor Quirrell admitted to Harry that he'd jinxed the broom and claimed he'd have gotten away with it too if it hadn't been for those meddling counter-curses. In other words, Snape tried to save Harry's life, and the only thanks he got was being set on fire. Harsh.
Voldemort's desire to beat death and become immortal was also why he and Quirrell ended up slaying innocent unicorns. Drinking the blood of a unicorn would keep someone alive – even if they were an inch from death.
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.
But Snape didn't have time to get too deeply into that. Later in the year, Dumbledore revealed that Harry was in fact a Horcrux, meaning that he would eventually have to die. Now, as firmly established, Snape was not the greatest fan of Harry, but that didn't mean that he ever stopped loving Lily.
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.
Draco tried to win Harry's friendship by encouraging him to stick to "the right sort" – i.e., rich purebloods like Draco. Harry's not a bigot or a snob, so he refused to shake hands with Draco, and the rest is history.
The scar is also a symbol of Harry's emotional sensitivity, because it hurts him whenever hatred is directed at him, as when Snape first sees him at Hogwarts or when Quirrell tries to grab him.
J. K. Rowling stated that the pain from Harry's scar on his forehead was caused by the piece of Voldemort's soul trying to leave his body through the wound it entered to rejoin its master's soul.