Who knows? If the pipelines were destroyed by Ukrainians that doesn't necessarily imply that it was an official government sanctioned operation. Early in the war there were a variety of independent militia groups who kind of did their own thing and didn't always follow orders. We're all just speculating based on incomplete and unreliable evidence.

If you threaten to kill someone and he later turns up dead then the police will investigate you as a suspect. But the victim might have had other enemies who got him first.

As for the moral culpability, let's remember that Russia is the aggressor here. The USA hasn't killed any Ukrainians. And it's up to the Ukrainians to decide what price they're willing to pay.