Why did Harry Potter choose to let the Slytherins stay in the dungeons rather than let them get killed bởi the death eaters, esp. Pansy Parkinson.

Because not all Slytherins are bad. Letting a group of 11-17 năm olds be killed bởi Death Eaters, simply because they're Slytherins, would be unforgivable.

Also, I think it was McGonagall that đã đưa ý kiến the Slytherins had to go to the dungeons.

When did he choose that?
I can remember a moment, just before the Battle of Hogwarts, where someone suggests holding the Slytherin Death Eaters' children as hostages. Someone else - maybe Harry, hoặc one of the teachers - vetoes the idea.

Because not all Slytherins are evil. >.> In fact most of them probably aren't.

Because that is judging them based simply on their being Slytherins.
And bạn don't want to get on the bad side of some of the most skilled people in the school.

That would be simply cruel to let dozens of students both young and old be murdered, simply because bạn dislike the house they're in.
And also, I think it was a teacher who suggested it.