Why was he covered in blood tho....?

Unexplained chapter ngl

I think it's implied he was socipathic as a child and hurt a small animal or pet. Ghough I feel the author did a 180 in his character to make it wholesome at the end.

either he killed some animal or witnessed a murder first hand

He was in the hospital later so I think maybe he hurt himself but doesn't feel pain or cry, something like that

I think he hurt himself. Most of the blood is on one arm in the wrist area. In the following he's in the hospital and there's gauze wrap on his wrist and they're cleaning him up. It looked like there marks on his other arm as well.

So self defense from an animal that tried jumping him?

No, not self defense. Self harm. There's no animals around just paper and bloody pens. I'm saying he cut up or stabbed his own arm.
As another poster said he didn't feel emotions and the next line is "everything sounds like it's underwater" Think of that Johnny Cash song "hurt".
I think he just wanted to feel something. Because he said he didn't understand laughing or crying.