Why did H.W. start the fire

I assume so . It was to draw attention to it .

First I didn't understand why he did that, but after the movie as I was thinking about it, did I realize that H.W. was looking at the "brother's" diary (or whatever that was), and probably he did realize he was an imposter, and that was his simple way of trying to solve the problem. So, what's tragic is that he actually tried to help his "father" and that's what got him into trouble.

He didn't start the fire. It's been always burning since the world's been turning.

It was probably more of a jealousy thing. His relationship with his father changed after losing his hearing and Henry was replacing him as the person Daniel cared for the most.

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I'd love to see you in the moonlight with your head thrown back and your body on fire.

I think H.W. was still in shock after becoming permanently deaf. And he was also upset that his father didn't have enough patience to understand his loss of hearing. It was almost as if Daniel couldn't acknowledge that his son was deaf. And also, before starting the fire, Daniel forced H.W. to drink a mixture of milk and whiskey (gross!) So maybe H.W. was just really drunk at the time.

The fire scene came right after Daniel told his "half-brother" about how he hated people & how he (the "half-brother") had that in him too. So I figured H.W. just inherited his father's hate for people lol

H.W. couldn't sign at the time of the fire either. It wasn't until he was sent away.