The reason that harry wasnt expelled is because Snape made the spell which dumbldore would be able to tell from the book which harry would have said that he found the spell from. Or that he cared about Lily too much and that makes Snape protect harry like he always has before.
Sectumsempra isn't crucio, avada, or imperio. It's not an unforgivable curse. Harry wasn't the one who used, or tried to use, an unforgivable curse in that situation. That would be Draco who did that.
Moreover, by the time Harry and others gained consciousness in the hospital wing, they were already surrounded by other teachers and the Minister, who were thinking Snape was the one who saved them. So it's also a matter of personal reputation for him, which was why he didn't punish them outright.
For using Dark Magic, albeit unknowingly, to cause serious harm, Snape sentences Harry to detention every Saturday until school ends.
Expelling Harry at that point would draw unnecessary light onto Hogwarts. At the time, Harry was the darling of the wizarding world. Him being expelled would have been noticed. That could have unintended consequences from the Minister getting his hooks into Harry to there being a loss of confidence from the public.
The darkest of the Dark Arts, Harry Potter unforgivable curses are the Cruciatus Curse, the Imperius Curse, and the Killing Curse. Although the curses are highly illegal, Harry using Unforgivable Curses happened, but he uses them once he was thrust into the Second Wizarding War.
Cruciatus Curse ('Crucio')
Harry tried to use this spell himself. The first time was against Bellatrix after she had killed Sirius. His attempt was unsuccessful because, as Bellatrix mockingly explained: 'You need to really want to cause pain – to enjoy it – righteous anger won't hurt me for long…'
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.
The short answer is no. While yes Snape did gash the side of James' face, Sectumsempra is a far deadlier curse. When Harry used the hex in The Half-Blood Prince, Draco collapsed onto the floor, and severely bleed through his face and chest.
There is likely a good reason that the Sectumsempra is not an unforgivable curse. It's mostly because Sectumsempra is Snape's personal curse that he created. It is not a widely known or used spell, and it is one that he kept secret — which means it can't be added to the list since no one really knows about it.
He strongly dislikes Harry and often insults him by insulting his father James. As the series progresses, it is revealed that his treatment of Harry stems from Snape's bitter rivalry with James when they were in school together.
Most probably not. Snape's hatred of Harry came from having to see James's face every day and Lily's eyes behind his glasses. Harry would've been a constant reminder of the love Snape lost, and I believe it would've been more painful if he was in Slytherin because they'd have to see each other more.
Because Voldemort literally took over the Ministry of Magic. And immediately after Dumbledore's death, Snape disappeared with the rest of the Death Eaters from the Astronomy Tower. The Ministry had a hard time tracking any of the key players in Voldemort's ring.
Snape saved Draco, and having realised that Harry got a hold of the old textbook, he punished Harry with a multitude of detentions for nearly killing Malfoy. Harry, despite disliking Malfoy, did not truly want to harm Malfoy to such an extent, and was both horrified and guilt-ridden by using the curse against him.
Vulnera Sanentur was the song-like incantation of a healing spell and counter-curse to the Sectumsempra Spell, which was invented by Severus Snape. While sectumsempra was invented by Snape, Vulnera Sanentur is often mistaken to be invented by him as well.
The Emerald Potion, also known as the Drink of Despair, is a mysterious potion which induces fear, delirium, and extreme thirst. According to Dumbledore, the potion cannot be penetrated by hand, vanished, parted, scooped up, siphoned away, transfigured, charmed, or otherwise made to change its nature in any way.
Harry Potter's use on Draco Malfoy in the Duelling Club. The Tickling Charm (Rictusempra), also known as the Rictusempra Charm, is a spell that causes a victim to buckle with laughter. It also has at least two other side effects on the victim. It is usually used as a diversionary tactic in duels.
The next year, during the Battle of the Seven Potters, Snape attempted to use the curse to sever the hand of a fellow Death Eater, but due to a slip, he accidentally cut off George Weasley's left ear instead, leaving him guilt-ridden.
James isn't motivated by anything much in attacking Snape in this way; he does it because he's bored, because he can, and because he wants to show off to some admiring girls, including Harry's mother, Lily. Harry is deeply disturbed by witnessing Snape's worst memory.
In Deathly Hallows, Hermione faced a real test of her physical and psychological stamina. After being on the run from a bunch of Snatchers, the gang were taken to Malfoy Manor, where Bellatrix proceeded to torture Hermione using the Cruciatus Curse.
In the books Bellatrix is clearly playing with her food. And in her overconfidence Bellatrix fails to deflect, dodge or block an immediately lethal spell from Molly Weasley. If you'd like to explain film Bellatrix's actions overconfidence prevented her from using Abada Kedavra straight away.
Why didn t the Death Eaters use Avada Kedavra all the time? Firstly according to the fake Moody Avada Kedavra (AK) requires a powerful magic. Therefore using it repeatedly would be magically - and perhaps even physically - very draining. Secondly Death Eaters don't always want/need to kill their targets.
Based on Harry Potter lore, and an article by The Healthy Journal, Draco's signature spell is a N.E.W.T standard spell, The Protean Charm.
Avada Kedavra
It's used to kill people, it's an Unforgivable Curse, and it was Voldemort's favourite, so it's going last. There's probably a big philosophical debate we could get into about violent curses like this – would this one have been okay to use on an evil dark wizard, for instance?