1) They are running out of time on this war of aggression, and this will force gas price action NOW, not in December. They can sell gas through other pipelines to the EU if it accepts it.
2) Norway just TODAY inaugurated a new pipeline to the EU. Norway has warned about unaccounted-for drone activity around it's oil and gas fields. Russia is likely sending a message: "Nice pipeline you have there - it's be a shame if something happened to it".
3) Russia has favorable actors planted in just about every western democracy, and some of them are already tweeting accusations against the US. As long as the Russians can keep suspicion off of them with reasoning like "what motive do they have?" and "somebody else must have done it", it works very much to their advantage in undermining opposition to their war of aggression.
In contrast, this is the last thing the administrations in the US or Germany need. They need stability and moderation of gas prices. Their methods are not to create chaos and try to take advantage. That is Russia's standard modus operandi.
So, yes, Russia has the biggest motives of anyone. They also have the means with extensive submarine capabilities, and ports on the Baltic Sea.