However, in a sudden turn of events, Snape chose not to fire but only defend himself. What remained unnoticed was — how he deflected McGonagall's spell killing the two Death Eaters standing behind him, before flying away to make sure that everyone around would stay safe.
But what McGonagall doesn't know is that Snape is actually a double agent working for Dumbledore, so he has no intention of harming her. That being said, Snape holds back quite a bit. Many fans, however, like to speculate who would have won the duel had Snape given it his all.
He didnt want to hurt professor mcgonagall because he was one of the good guys after all but he couldnt blow his cover with Voldemort either. Snape, is the greatest and more complex character of the story and without him (and Allan Rickman) the story would pretty much fall apart.
Escape was his best option. McGonagall believed him to be an agent of Voldemort and was no doubt prepared to duel to kill. Snape had no wish to kill McGonagall or to die. Moreover, his tasks were not yet complete.
Snape was then chased by McGonagall, Sprout, and Flitwick into a nearby classroom, jumped through a window and flew away, a skill he presumably learnt from Lord Voldemort. Minerva contemptuously called Snape a "coward".
Harry Potter and the Half-Blood Prince
However, his theory was dismissed. 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.
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.
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".
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.
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.
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.
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. Instead of providing Harry a safe haven from his abusive home with the Dursleys, he tormented Harry over an old grudge.
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.
Why didn't Dumbledore tell Snape about the Elder Wand so Snape could break it before Voldemort found it? The answer is simple: It was far too risky for Severus Snape to be aware of both the Hallows and the Horcruxes.
Knowing that Snape has been on the Dumbledore's side for all seven of the Harry Potter books reveals him as an extremely brave man and a truly skilled spy. Snape has had to face and deceive the terrifying Lord Voldemort constantly.
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.
McGonagall followed up Harry's action by using the Imperius Curse on Amycus before tying him up with a net, rendering him useless during the Battle of Hogwarts.
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.
Professor Slughorn, his Potions Master, was very fond of both Snape and Lily, so they were both likely a part of his elite student group, the Slug Club. However, it's never clearly mentioned in the books, probably because Slughorn only liked to show off the students he was most proud of.
“You disgust me,” said Dumbledore, and Harry had never heard so much contempt in his voice. Snape seemed to shrink a little, “You do not care, then, about the deaths of her husband and child? They can die, as long as you have what you want?” Snape said nothing, but merely looked up at Dumbledore.
“Dumbledore didn't want to lose his wand at that point and Draco disarmed him. So that meant that the wand gave Draco its allegiance, even though Draco never knew it, even though Draco never touched it. “From that moment on, that wand gave its allegiance to Draco, and it wouldn't work as well for anyone but Draco.”
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.
The confusion for audience members who have only watched the movies lies in Filch's dialect; although “ma'am” rhymes with “ham” in Standard American English, in some dialects of British English is rhymes with “farm.” Furthermore in some dialects of British English “r”s are deleted, so that pronouncing “ma'am” to sound ...
Yet, Sirius freely admits he has not heard a rumor Snape was a Death Eater. The answer seems to be that Snape as a Death Eater was not particularly enthusiastic even as a teenager. Malfoy like Bella accuses him of being all talk and no action as a Death Eater.