He saw red eyes and did it by reflex! Now masters gonna think Kyon scared of him! Noooooo...

He was facing away and his eyes were glazed...?

Nope, you can see that the symbol on his neck changed to intron's, and his eyes were not focused before he slashed Lucaon, so he was probably controlled or posessed.

But it shows him whirl around and see Lucions eyes even tho he was dizzy. I just thought he was out of it and attacked based on that. I don't know much about the mark tho.