Honestly I think we are among the first ones heavily theorizing around here, plus vocaloidsrcool, but I do believe after the next update people will share more of their theories once the real mystery starts to surface.
-- Warning, heavy themes below:
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And regarding your comment: the developer did say the next update is going to be more brutal, which is definitely going to be something. I think we might see Myrddin getting more directly hurt, yes. But I also believe he tried to harm himself, after all he has been there for a long time.
We don't know the exact time. He was a child when he arrived but he might age differently. Myrddin did say he created a clock and he destroyed it when it got to the 1000th day. So he stayed there for AT THE VERY LEAST almost three years. Of course, not counting the fact that he took a long time to actually know how to create objects on his own to finally create the clock. So in all that spare time, I'm sure very negative thoughts crept inside his head.
But honestly, I think the motivation behind his actions might not only be the isolation but the torture the Voices do to him. He comments something about there being no limit to what one can harm the other, even though he says he is resilient. So I think he is partially referring to the torture he goes through because of the Voices. We did see the Voices use loud noises, change the room size, and sever our connection with Myrddin. So they probably can do a lot more.
Random note: also, very interesting that Myrddin does not like brown, if I'm not mistaken, especially because the whole room is pretty much white and brown. Very curious.