I happen to support the Ukrainians in this war, both because Russia invaded them as opposed to vice versa and because my heritage includes Ukrainians from around Kiev on my maternal grandmothers side.
But fairness makes me point out that the question is not if any Ukrainians were in Russia killing Russians, but if Ukrainians were killing Russians in Ukraine. The ethnic Russians in some of the provinces of Ukraine were being killed and were trying to separate from Ukraine and go back to being part of Russia.
Fairness also makes me point out that this was also the excuse the Russians used in 2014 to invade Ukraine and take back the Crimean area. Seems to be a pattern starting here. And for anyone who wants to answer claims that it is all Trump's fault, I would point out that in both cases with Russia, there were democratic presidents in office. And when Germany used the same excuse to invade Czechoslovakia, it was also with a Democratic president in office.