The enemy invaded and you embraced him, now that we got you back from him you want mercy?
That was a situation of Kiev's own doing though, trying to ethnically cleanse eastern ukranians and anyone who spoke russian since 2014.
It would be like expecting the chechens to not embrace an invading army in the 1990's, should that have happened.
We shouldn't forget this war is just an extension of the 2014 nazi coup.
It will send a warning to the whole country. Think very carefully before becoming a traitor otherwise when we get the city back you will suffer, your property seized, thrown into prison/hard labour/fired from job and then ostracized.
Ukranians were already doing that before the war. What does change is that they can probably do mass executions and the west will cover it all up.
Downside for Ukranians?. They'll face the same type of insurgency in the East Russia would face if they would have tried to occupy the West.