In the film, Snape is forced to fight Professor McGonagall after Harry Potter reveals that he was the one that killed Professor Dumbledore (at Dumbledore's bidding, though at the time the fact was unknown).
Because Minerva was his friend and they were fighting on the same side (even though she didn't know). So, even when Minerva was duelling to kill, he wasn't, and if he had kept on fighting he might have been forced to hurt her.
If at full capacity, defending everything she held dear, the new Headmistress McGonagall was unable to defeat Snape. It suggested the latter was the more powerful professor.
After Snape threatens to severely punish every student who fails to tell him about Harry's whereabouts, Harry reveals himself and members of the Order of the Phoenix back him up. Minerva McGonagall, however, steps up and singlehandedly fends Snape off using a fiery spell.
Because Snape ran away from their fight. OK, he flew away. The point is, he retreated rather than finish the fight. Professor McGonagall didn't know all the details, just that someone she had patiently waited all year for the chance to beat his ass didn't even give her the satisfaction of a proper duel.
So Snape was cruel, biased towards his house, and downright rude to his students: this we can't deny. Calling poor Hermione an 'insufferable know-it-all', for example, was pretty terrible.
Later in the year, McGonagall duelled with Death Eater Alecto Carrow during the Battle in the Astronomy Tower and later learned of Snape's murder of Dumbledore. McGonagall was initially appointed as Headmistress, but was demoted after Voldemort took control of the school and placed Snape as Headmaster.
Professor McGonagall is a Death Eater
According to the theory, McGonagall's seeming disregard for Harry Potter's safety, her talent for transfiguration, and and her stoic nature all point to the fact that she was a Death Eater.
Professor McGonagall knew that Snape was working with Dumbledore. Everyone in the Order did, which is why Snape was actually let into Order meetings - if they didn't know, they'd likely have tried to not let him it. However, she didn't know that Dumbledore and Snape planned Dumbledore's death.
Weaknesses. McGonagall is never portrayed as having any particular weaknesses. Her stern facade likely prevents anyone from getting close to her emotionally, with the possible exception of Dumbledore. We are led to believe that she prefers her solitude.
Sure McGonagle is more trustworthy in the traditional sense when it comes to her job but Snape was more trustworthy to be triple agent. Snape was more inscrutable. He had the dirt on Snape (he could dangle his feelings towards Lily to get him to agree to anything). Snape could play a double agent more convincingly.
Draco had several reasons for lying to Bellatrix, including the fact that he was never a bad person. Moreover, he no longer found working for the Dark Lord appealing and hated how Voldemort treated his family. He did not intend to harm anyone and believed Harry was the only person capable of defeating Voldemort.
Winner: Snape
The kind of versatility in magical abilities that Snape displayed was almost around the level of Albus Dumbledore and Voldemort, making him easily more powerful than the average wizard and even the members of the Order of the Phoenix and the Death Eaters.
Pursuing, McGonagall screams, "Coward!" Uncloaked, Harry and Luna rush inside, startling the other professors. Snape has leaped out the window. Harry thinks he must be dead, but McGonagall bitterly comments that, unlike Dumbledore, Snape had a wand and has learned a few tricks from "You-Know-Who".
Neville Professor Snape Neville fears Professor Snape who treats him rudely with sarcastic comments. Neville seems afraid of this harsh treatment that makes him feel inferior constantly. Parvati Blood-stained, bandaged mummy Parvati is afraid of something that is scary to her, possibly because it can harm her.
Actually every one were very much aware about the plan to keep Harry safe, but no one really knew the extent to which Snape is on that plan. So, Dumbledore kept it secret because he thought it is essential for the safety of Harry Potter.
Harry watched Hagrid getting redder and redder in the face as he called for more wine, finally kissing Professor McGonagall on the cheek, who, to Harry's amazement, giggled and blushed, her top hat lop-sided.
Dumbledore himself was aware of Snape's actions and trusted him to aid him in his mission. He also shared some information with Harry regarding Snape's true loyalty and role in his plan, but only toward the end of the series.
As I said earlier, many Hogwarts professors are arguably more powerful than Voldemort. Minerva McGonagall is, without question, one of them. A true Gryffindor, McGonagall is afraid of nothing, least of all Death Eaters or the likes of Dolores Umbridge.
The Death Eaters and the Dark Lord don't simply cast Avada Kedavra on everyone all the time because it doesn't suit their long-term goal of ruling the world and having a wizard-run society.
Definitely not. I still find it to be intriguing given the amount of evidence which supports the conclusion that Hagrid is one of the top servants of Voldemort.”
He did not even wish he had chosen differently — he believed he had made the only reasonable choice. He felt what he said, regret. Because he had to kill his most useful servant. His other Death Eaters either fled or were imprisoned when Voldemort was 'half-dead'.
He was the one who told Voldemort about it, and so when it became clear that it referred to a baby and that this baby was the son of James and Lily Potter, Snape knew about it first-hand.