Why Did Mcgonagall Send Slytherin to the Dungeons

Why is the Slytherin common room in the dungeons?

Each Founder would be able to choose where they wanted their house common room to be, and there are reasons that Salazar would have chosen the dungeons. The quiet of the dungeons would distill the mind, and ensure that the Slytherin students would remain relatively undisturbed by the other Houses.

Did any Slytherins fight for Hogwarts?

They may not have been fighting by his side, but they were still fighting ON his side. So it would appear that each and every Slytherin student fought for Voldemort bar none, and the only Slytherins who fought against Voldemort were the teachers – Slughorn, openly, and Snape, covertly.