hzv, again, has positive, creative connotation. The example you offered,
Hos6:5, only proves my point:
I hewed (hzv) them by the prophets
Prophets didn't kill or cut to pieces, they hewed people into good Jews.
Now, returning to Is51:9,
mahzevet is causative of hzv, so, made the one to hew, therefore,
Are not you the one who made the rahab (Egypt) who hews the stones (made
pyramids?), [and the one who] shocks the crocodile (pharaoh; makes him
tremble, or empties)
The meaning is that the power of Almighty is two-folded, to give and to empty
of, to help and to make tremble.