I can see the US doing it as they've been vocal opponents to nordstream since its inception, I can see Ukraine wanting to do it although I doubt they'd have the resources, might also have been some other rogue European faction wanting out from under Putin's thumb.
Permanently shutting it down significantly constrains options for anyone who might seize power in Russia next.
Destroying the pipelines removed the potential reward for an internal rival to replace him.
Then the explosions happened, which prevented gas from being transported through the pipelines - except for one Nordstream 2 pipeline, which actually would require Germany to budge for it to be operational. Russia even stated that they'd be happy to send gas through the remaining pipeline as soon as Germany backtracked.
Whether or not you think Russia did it, the explosion had the effect of turning something the Russians had been trying and failing to convince other countries of into a reality.
Putin's rivals make Putin look soft. If they do take power, they will end the conflict quickly and definitively.
Indeed, Nordstream hadn't been running gas for about a month at the time of the explosions. (Indeed, Nordstream 2 also never ran gas). That is critically useful information for assessing who had motive to blow up the pipeline, yet everyone speculating on the matter seems to assume that it was being used at the time of explosion.
Heck leveling Kiev will do that too. Could be done in a day.
Meanwhile, all the rhetoric of Russia “saving” a brotherly nation goes flying out the window.
Of course it will.
>ground troops from neighbors, EU, and possibly NATO
They are not suicidal, I don't think.
>Russia “saving” a brotherly nation goes flying out the window
Yes. That is the reason war will continue the way it is now: very slowly, and stupid.
Edit: reddit spacing
I'm afraid something as drastic as the annihilation of Kiev will lead to actions that are beyond the usual risk assessment levels. Countries will be compelled to act, (repeated...) threats of nukes be damned. Europe will not tolerate another Nazi Germany on its borders, period.
Put another way, a massive, discontinuous step in escalation will inevitably lead to a similar step from the other side. There is no world in which Germany and Poland go "OK then" and withdraw all aid.
Ukraine's military barely held on against 90k professional soldiers and 140k mobilised. It would not stand a single chance against 3 million soldiers and a fully militarized Russian economy. Russia hasn't even called up a tenth of its trained reserves.
Putin's concern would be the home front.
No, they just don't have the means to escalate this any further (without using nukes).