Why is Russia trying to annex it?

Oh I don't know maybe because the Kiev regime had been telegraphing for eight years their plans to use the oblast as a vector to reconquer Crimea

Why is Russia trying to annex it? Why is the russian military leveling towns, sometimes still with civilians living there

You think the Russian military are the one leveling towns and territories they'll have to rebuild and administer, filled with allied civilians? Motherfucker, the Ukrainian army are the ones openly leveling towns to stop any and all Russian advances, angering western advisors by wasting mountains of artillery ammunition while hunting "collaborators" in the regions they're purportedly trying to liberate, only Bakhmut can be said to have been destroyed in equal measure by the Russians, and that was a result of Ukraine turning that town into Redux-Stalingrad for no real strategic reason beyond media glazing

You literally don't know anything about this conflict

The people of Kherson don't want to be part of Russia. Why is Russia trying to annex it?

Russia is building a buffer zone to push the site of potential NATO bases and weapons systems further and further from Moscow, and to be able to militarily hold that buffer zone (requiring infrastructure, transportation, continuity with Crimea which was annexed for similar reasons, etc.). It views NATO expansion and encirclement as an existential threat, for pretty good historic reasons.

You're right that its intentions aren't altruistic as other users are claiming/implying. It is a nation-state. But its intentions are predictable and inline with those that basically every other nation-state would have in the same situation, because that is the nature of the institution.