Because they aren’t going to stop. See when they took Crimea in 2014, now it’s a full invasion.

You don’t try to appease fascists.

Because it’s not a “small” portion they WANT. That’s what’s known as a “beachhead”. Putin and the rest of the Duma have signaled that they want UKRAINE, in its totality, and preferably would not like those people with their ideas of “independence from Russia” in their heads - hence Putin actually saying the only victory he would accept is the destruction of Ukrainian identity as a concept (war against a concept doesn’t end well - see “war on terror/war on drugs”.) This is also ignoring the fact that Russia ALREADY tried the “storm in, kill the head of state, annex the country” bit with Georgia back in 2008, and got their shit kicked in by the Georgian military. Expansionist powers like Russia don’t just stop with whatever country they’re currently invading - there’s always another “over there” to go conquer.

If you want more evidence on this, I highly recommend you read up on a little thing called Imperial Japan, or potentially just Manifest Destiny. There’s never such a thing as “enough” for a state hellbent on expanding at all costs into territory they think is rightfully “theirs”.