In There Will Be Blood, why did HW go mute and try to burn down the shed?

Firstly

The accident did not initially rob H.W. of the ability to speak. Immediately after his injury, Daniel runs H.W. down to the commissary building (where the crew would eat), lays him upon a table and asks where he was hurt. H.W. replies "I can't hear my voice." This dialogue can be seen in the following clip:

There Will Be Blood Clip Oil Well Explosion (HW goes deaf)

As time goes on, since H.W. cannot hear himself any longer, he loses the ability to be understood, since he can't tell if the sounds he is making are accurate or not. Eventually, he stops using his speech altogether, as it no longer has any meaning to himself.

Secondly

Now, on page 90 of the script, the fire scene is described. H.W. had already figured out Henry wasn't Daniels brother earlier. He pours oil on the floor around Henry's bed, and then in a trail out to the main room. He strikes a match to the oil trail, and:

It takes off like a rocket, shocking/terrifying H.W. and in a flash, Henry's room is on fire.

The fire quickly spreads up the curtains and to the rest of the house. However, as written, H.W. attempts to kill Henry, not Daniel, only he doesn't realize the danger of crude oil and a wooden house. The oil startles him with how fast it ignites and there is no time to recover, as the rest of the house is quickly engulfed.