why does legree cry out and say " what have we to do with thee, thou jesus of nazareth? - art thou come to torment us before the time"?

Legree can no longer break Tom's spirit because his heart is in the hand of God. Legree no longer had any power over Tom because Tom was protected by a higher power, a power Legree could not fight against.

"But the blows fell now only on the outer man, and not, as before, on the heart. Tom stood perfectly submissive; and yet Legree could not hide from himself that his power over his bond thrall was somehow gone."