The reason is resentment rooted in an inferiority complex. Russia's state ideology is that they're being oppressed by "the West", and they really believe it up to the highest levels of government. They're quite convinced that hurting their "enemy" this way amounts to securing their interest, because by damaging "the West", their own relative power is elevated and that makes them a serious player on the international stage.

Indeed many people who subscribe to Rational Actor theory of politics are stupefied by this. However the thing about personalistic dictatorships is their foreign policy and aggression are all subject to the whims of one person, even though the media still refers to them as if they are real countries with collective decision-making. And they indeed may be acting rationally just not in the axiomatic framework any sane person would even consider.

Say some dictator lived through a trauma that he projects onto some group of people. Or that he considers himself a spiritual successor (perhaps even the reincarnation) of Ivan the Great, the Collector or Lands. Once you ease yourself into this mindset you see the logic.

Russia is desperate for leverage. Apart from China, they are severely isolated from rich markets. Targeted violence is an attempt at generating leverage, in the same way some racketeers would hit shops that refuse to pay "protection" money.

I think sabotage is at least partly signalling: "Don't support Ukraine too much, or more bad things will happen to you."

The point of the "special operation" was that there would not be a culturally-adjacent functioning democracy next door, because that might give the Russian people ideas.

Creating chaos in democracies is forcing the elected politicians to focus on the interior, thus leaving them less resources (or public interest) for stopping remote invasions.