Ukraine is still pumping Russian gas to Europe through their own pipeline and are being paid by Russia for it

They could sabotage their own if they really wanted to, or just switch it off.

For Ukraine, it's better to confiscate NS as a compensation for the war, so it better to keep it off and intact, until it will be Ukrainian.

Because Ukraine isn't going to get caught committing an act of war against NATO in Europe while fighting an existential war for its own existence against Russia while completely dependent on supplies from NATO.

The obvious answer here is going to be the correct one: it was Russia. Specifically Putin: Russia can be rebuilt on gas to Europe if it ceases the invasion, but Putin specifically won't survive such an event.

Ensuring his potential successor can't "make a deal with the West" is exactly his sort of thinking. With a side benefit of people asking questions like this right now (I mean, this is catastrophic for him long run, but so is the entire war so it's also on brand with his incompetence).