They couldn't turn off Nordstream 2 because the Germans stopped certification, so it was never turned on. And they already had turned off Nordstream 1 months ago, but that failed to change the West's behavior. So they had no more Nordstream cards left to play.
As for why Putin would blow it up -why did Cortes' burn his own ships, when all he has to do was order his men not to sail away?
I am not sure who blew up the 2 Nord streams. But Bad Vlad is a contender. IOW there are others who might have, and Russia is one of them. Most here count Russia out. All that crazy nuke talk from Vlad and his media stooges, this tells me Vlad is the type who would blow up his own gaz pipeline. That is half owned by Germany, half by Russia.