Definitely support.

In the Defense tree:

Deterrence will only add roughly 2-3 extra attacks for the entire fight,depending how long your line up is lasting. Every 3rd CB attack is a stun, He is likely going to start killing your team from the 20th turn point so he will use his stun attack 8-12 times and 20% of the time the ability will proc IF the stun isn't on Nethril himself.

Retribution is fairly good but doesn't really come into play until about the half way point when CB actually starts doing enough damage to register.

Charged Focus is always up on Nethril since you won't be using any of his CD abilities, so that frees up some space for more Speed or HP for durability.

Arcane Celerity is pretty huge on Nethril too since he is placing A LOT of poison debuffs. You can pretty much count this as a flat 10% speed buff.

4th tier is mostly a waste for Nethril. You can make use of Lore of Steel if you're going for Speed and Acc sets but the meta for end game leans towards Lifesteal so depending on your artifact choices and your team line up will determine what you spend points on here. You may like Evil Eye if you use him for Dungeons or Arena.

And finally Sniper and Master Hexer, You can basically count these as roughly 10% extra poison damage each, which is pretty huge for Nethril.

Definitely support.

In the Defense tree:

Deterrence will only add roughly 2-3 extra attacks for the entire fight,depending how long your line up is lasting. Every 3rd CB attack is a stun, He is likely going to start killing your team from the 20th turn point so he will use his stun attack 8-12 times and 20% of the time the ability will proc IF the stun isn't on Nethril himself.

Retribution is fairly good but doesn't really come into play until about the half way point when CB actually starts doing enough damage to register.

In the Support tree:

Charged Focus is always up on Nethril since you won't be using any of his CD abilities, so that frees up some space for more Speed or HP for durability.

Arcane Celerity is pretty huge on Nethril too since he is placing A LOT of poison debuffs. You can pretty much count this as a flat 10% speed buff.

4th tier is mostly a waste for Nethril. You can make use of Lore of Steel if you're going for Speed and Acc sets but the meta for end game leans towards Lifesteal so depending on your artifact choices and your team line up will determine what you spend points on here. You may like Evil Eye if you use him for Dungeons or Arena.

And finally Sniper and Master Hexer, You can basically count these as roughly 10% extra poison damage each, which is pretty huge for Nethril.