Someone finds a wand for Dean from somewhere. It had been taken from a Slytherin as someone fled to the Room in the first place, and he really doesn’t want or need to know who. It works and that is enough for him. Out of anyone, he knows how important it is to win this fight. He still has nightmares about those months on the run and he hopes that every Death Eater he takes down will lessen them a little.
He is incapable of remaining above their level, and the masks hiding their identities start to hide their humanity entirely. They stop being people and start people numbers, just like Dean had been on the run. Counting them up, he doesn’t hold back. Stunned Death Eaters can get back up and kill him before Dean eve realises they have come to. He kills them; he does what he has to.
He recognises one man he takes down as one who had been five years above him in school, and who had pushed him into a wall during his first week. It is still not enough to bring up any emotions in Dean other than anger and the clawing desperation that pushes him on. He has enough of running. People had run from the truth for years, allowing the racism that was destroying their world to continue to fester, and people had to stand up and say no more. Dean was prepared to be one of them.