Why did Moses used 2 different words in Genesis 6:7 and 17 when writing about the consequences of the Great Flood?
Not a complete annihilation
The first term refers to an action against man and the animals - exterminating or eradicating the individual creatures currently living. ‘Exterminating’ does not wipe out a species - it only removes the living creatures.
The second term refers to an action against flesh in general - ruining flesh as a whole. To ‘ruin’ something is also not to permanently remove it, but to take away its ability to function as before.
The impression I get is of intention:
Extermination: living creatures not species
While God sought to eradicate the actual, individual creatures that were currently living, He had no intention of eliminating any species in general.
This is demonstrated by the ark, which protected one breeding pair of each species.
Ruin: influence not existence
While God sought to destroy or ruin the influence that ‘flesh’ wielded on the earth, He had no intention of eradicating or exterminating flesh altogether, let alone eliminating it.
This is also demonstrated by the ark, which protected certain flesh under the influence of Noah and his family (who listened to God instead of being led by the flesh).
So the intention behind the Flood was not to annihilate species or flesh in general, but to remove the influence of ‘flesh’ (evolutionary instinct, fear, hatred, anger, etc) on the world, and provide a new opportunity for the Spirit of God to lead the way through Noah.