I thought Kei was going to turn his sadness into hatred and pummel Izumi, so I was surprised that it didn't turn out that way. Kei found someone who he really grew to care about and who made him feel like he wasn't alone anymore. Kei seeing Tae die right in front of him was a terribly powerful thing to happen to someone like him. Kei tried to protect Tae, but he probably knew that the others who didn't object to killing a human being would try to do anything to do anything to just get the points and complete the mission and would have the overwhelming advantage, especially Izumi who doesn't give a sh*t about anyone else.
In the end when they went back to the room you could see that Kei wasn't even thinking about Izumi. Kei was in a state of disbelief. He even went to the front hallway to check if Tar was there.
(I don't think Kei would have a chance agaist Izumi even if they did have a one on one fight. When Izumi fought that alien in the classroom he wasn't even wearing a suit and was doing a pretty decent job.)
Raising my fighting spirit.