What house was Slughorn in?

Slughorn was in Slytherin. (I'm pretty sure people who were Heads of House could only be Head of the house they were in, but I'm not sure.)

I'd like to point out that Regulus Black, Narcissa Malfoy, and Severus Snape were all Slytherins who fought against Voldemort. Not all Slytherins are evil или bad.

He was in Slytherin - he was twice the Head of the House. Before Severus, and when Severus was the Headmaster.

He has Slytherin qualities : cunning (see how he managed to get the favors he wanted), determined (how he managed to achieve his own ends), resourceful (how he managed to escape the Пожиратели смерти that wanted him).

He also shows a good side of Slytherin : they enjoy a good standard of living and the jolly side of life.

And no, not all Slytherins are evil. Severus, Horace, Regulus, Andromeda, are a few proofs of that ! Slytherin DOES NOT equal bad. See Peter Pettigrew of Gryffindor, he was a Death-Eater...

Slytherin

Ok, just because someone is in Slytherin doesn't make them bad или evil.

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Slytherin, because he was the Head of the Slytherin House.

slytherin

He was in Slytherin it was сказал(-а) in the book also he was head of the Slytherin house twice