Why weren't Muggle-borns killed at Hogwarts after the end of Half-Blood Prince?

Question

Wasn't Colin Creevey a Muggle-born? How did he survive to play a part in the Hogwarts battle? Did he not attend Hogwarts then? Or were the Carrows simply stupid?

Answer 1

Because the Dark Lord didn’t allow Muggle-borns into Hogwarts.

Under the Dark Lord’s rule, students had to prove their blood status before they were allowed to attend Hogwarts. They couldn’t get in until they’d been certified pure enough by the Ministry.

“This way, Voldemort will have the whole wizarding population under his eye from a young age. And it’s also another way of weeding out Muggle-borns, because students must be given Blood Status – meaning that they have proven to the Ministry that they are of wizard descent – before they are allowed to attend.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 11 (The Bribe)}}

Therefore, none of them were at Hogwarts to have been killed there, they were removed before.

Colin Creevey won’t have attended, he came with Dumbledore’s Army.

Colin Creevey wouldn’t have been attending Hogwarts because he was Muggle-born and wouldn’t be allowed to - but he came back with the rest of Dumbledore’s Army when they were all called.

“We sent messages to the rest of Dumbledore’s Army,’ Fred explained. ‘You couldn’t expect everyone to miss the fun, Harry, and the DA let the Order of the Phoenix know, and it all kind of snowballed.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 31 (The Battle of Hogwarts)}}

The members of Dumbledore’s Army weren’t all attending Hogwarts at the time. Some of the members who came were too old to have been, like Cho Chang, and some were previously in hiding, like Dean Thomas. Colin would have come in when Dumbledore’s Army was all called.

Muggle-borns were sought and put in Azkaban - some were killed.

Though they weren’t killed at Hogwarts because they didn’t get to attend, they weren’t in any way “spared” or “safe”. Muggle-borns were told to present themselves for interrogation by the Ministry.

“The Ministry is determined to root out such usurpers of magical power, and to this end has issued an invitation to every so-called Muggle-born to present themselves for interview by the newly appointed Muggle-born Registration Commission.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 11 (The Bribe)}}

If they couldn’t prove they had a wizarding relative, they were punished by being sent to Azkaban.

“Nevertheless, unless you can prove that you have at least one close wizarding relative, you are now deemed to have obtained your magical power illegally and must suffer the punishment.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 11 (The Bribe)}}

Proven Muggle-borns were sent off to Azkaban, if they weren’t given to the Dementors first. Arthur Weasley, who still worked at the Ministry, knew the charged Muggle-born he referred to was there.

“So, Dirk Cresswell is ten times the wizard you are,’ said Mr Weasley quietly, as the lift sank ever lower. ‘And if he survives Azkaban, you’ll have to answer to him, not to mention his wife, his sons and his friends –”
_{^{- Harry Potter and the Deathly Hallows, Chapter 13 (The Muggle-born Registration Commission)}}

They were, at least sometimes, killed if they didn’t comply. Troublesome Muggle-borns were sent to get the Dementor’s kiss, effectively killing them by sucking out their souls.

“This is your final warning,’ said Umbridge’s soft voice, magically magnified so that it sounded clearly over the man’s desperate screams. ‘If you struggle, you will be subjected to the Dementor’s kiss.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 13 (The Muggle-born Registration Commission)}}

This wasn’t an empty threat designed to scare - the man was indeed handed over to them.

“Take him away,’ said Umbridge. Two Dementors appeared in the doorway of the courtroom, their rotting, scabbed hands clutching the upper arms of a wizard who appeared to be fainting. They glided away down the corridor with him and the darkness they trailed behind them swallowed him from sight.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 13 (The Muggle-born Registration Commission)}}

Being in hiding didn’t keep Muggle-borns safe either. The Ministry actively sought them out, and paid rewards to anyone who turned in a Muggle-born to them.

“Snatchers,’ said Ron. ‘They’re everywhere, gangs trying to earn gold by rounding up Muggle-borns and blood traitors, there’s a reward from the Ministry for everyone captured. I was on my own and I look like I might be school age, they got really excited, thought I was a Muggle-born in hiding. I had to talk fast to get out of being dragged to the Ministry.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 19 (The Silver Doe)}}

The Muggle-borns were actively tracked down to be removed from wizarding society by any means, including being killed. The Dark Lord didn’t consider them worth mercy. In the official Ministry position, they were said to have stolen their magic.

“Recent research undertaken by the Department of Mysteries reveals that magic can only be passed from person to person when wizards reproduce. Where no proven wizarding ancestry exists, therefore, the so-called Muggle-born is likely to have obtained magical power by theft or force.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 11 (The Bribe)}}

They weren’t considered “magical blood” so weren’t worth saving - the Dark Lord and the Death Eaters had no qualms about killing them off.

Answer 2

No, there was no systematic genocide of Muggleborns after Voldemort's rise in the DH. Yes, some Muggleborns and random Muggles were killed, as a form of tactical terrorism.

Voldemort was evil, not stupid. He wanted the wizarding world to slowly become fearful of him to the point where he could rule over them. Notice, the government wasn't taken over until Bill and Fleur's wedding, with the assassination of Rufus Scrimgeour and the appointment of Pius Thicknesse.

Before that point, Voldemort and his Death Eater followers (either willingly or under the Imperius Curse) were gradually seeping into power. There was simply no need for killing Muggleborns openly just yet, lest risking a kind of martyrdom uprising from the wizarding world (openly killing kids studying in one of the most infamous wizarding schools would be the last thing on Voldemort's mind).

You could also argue that Voldemort had some "humanity" in the sense that he was hesitant to shed "magical blood" unless it was necessary.

Answer 3

Because there was no reason to, it was dangerous and it was difficult

They had come under strict orders from Voldemort: Malfoy was to kill Dumbledore, and then leave. Of course, the Dark Lord didn't expect Malfoy to succeed, but the plan was more or less the same. Why? Although Voldemort wanted to kill off all Muggle-borns, he planned on doing it sometime after. And the Ministry could show up any second if they hung around long enough to attack Muggle-borns because by then someone would have let them know. And lastly, the only Muggle-born in the corridors that night was Hermione, and she could defend herself. The rest of the Muggle-borns were in their dormitories, and it would be difficult to know the Gryffindor password, Ravenclaw riddle answer, and Hufflepuff barrel rhythm. Hufflepuff even has a way to catch intruders:

An enchanted stack of barrels, which will only open the door if the correct barrel is tapped in the correct rhythm. If done incorrectly, a barrel will burst open and cover the intruder in vinegar

All three of these and possibly more reasons are why the Death Eaters didn't murder any Muggle-borns that night.