Kyon.

i bet it was the collar (⊙…⊙ )

I thought he was just surprised and just acted and thought he was a Zero

It was his zero eye I don't think he did it on purpose

No. He wasn't frightened and his eyes had a glazed look.

Look at the page where one of the zeroes touches the back of Kyon's neck.

Idk if you noticed in the scene when Kyon fell, but before he hit the ground, you see some sort of memory of his, which is the image of someone's hand and Kyon. On the back of his neck, he has the image of the two-headed eagle thing, instead of the mark he has from Lucaon. That eagle image is the mark of Intron, which is the organization that Ryan suspects of being the ones who killed Lucaon's parents. They're also known for turning their own members into Zeros and controlling them to attack others (and possibly allow them to use some of their supernatural powers, like when Karis was attacked and had been temporarily blinded by the Zero).

A possibility is that before Kyon started working for Lucaon, he was given the mark but had his memory wiped. Then he was sent as a sort of spy without knowing. The friend who mentioned the job to him could have been a member of Intron and told him about the job so that Kyon would end up working for Lucaon.

So what might have happened in the recent chapter was, whoever had given Kyon that mark (some higher-up in Intron maybe?), might have brainwashed Kyon for a short moment and made him attack Lucaon. I don't think it was the Zero who gave him the mark because the hand was just a normal person's hand, while Zeros are like completely black. Also because Kyon was already falling, but the image of the hand touching his neck had him standing up straight, so...yeah.

Could be wrong though. I'm just throwing stuff out here. There's no explanation of how Kyon could have two marks and the first one not being noticed, so we'll have to see with the next chapters... :/

I think that time he suddenly remembered the first time he became a servant. I kinda saw the same scence on the raws i guess..

Oh WOW! That sounds so interesting!

Except that his eyes looked like he was being controlled and there was no reason for him to randomly slash Lucaon. I think it's the triggers of his memories that set things in motion. Just like I think it was with the bloody papers flying around. So he may have just been remembering something in that scene but it might have been enough to activate the mark of his former masters.

So, he had a former master? Ahhhh. I'm just so curious!

Guys, what if, now this is just a theory, but. What if in chapter one, Kyon is cutting off his collar because it overlaps the Intron mark? The following line after that is, "I should have never gotten caught." So he regained his memories and went back to Intron, removing Lucaon's mark. I'm an idiot, I just realised.

what chapter was that?? was it the first chapters??

It wasn't chapter 0, but chapter 1.

Guess i have to re read ╮(￣▽￣)╭

You might be right but I wonder if instead of trying to cut the Selvior collar off he is instead trying to cut off the Intron mark? Maybe this incident allowed it to take him over again?