I don't think PB helped Finn at all. I think he is still repressing his
pain and it will come out. I think this was the point of the shadow of
Finn's missing arm. I think too that the deer was just as much afraid of
Finn, if not more afraid of Finn then of the height, which is why it
ran away from Finn.

The deer was truly terrified and Finn did nothing to help the deer. He just ignored it. For Finn to ignore the deers plight was sadistic. The deer was there because of Finn. He could have helped it, but he chose not to.