Which is actually not the case for most Ukrainians of Russian descent:

  Most of ethnic Russians in Ukraine signed for the 
  Ukrainian army, fighting against Putin’s invasion, 
  against the same Russians that came from the other side.

(from http://marginalrevolution.com/marginalrevolution/2017/05/con.... Note the source is Kasparov, who is not an entirely unbiased source where Russia is concerned ;))

>Note the source is Kasparov, who is not an entirely unbiased source where Russia is concerned

To say the least...

Well, it's a big question - what is real "ethnic" russian. It's like an "ethnic USA citizen".

Less so though, as USA was from the onset a nation of hodgepodge nationalities from everywhere -- and without a much older local culture (except of the Indians, which were genocided away).

It was the constitution/law that held US together as a nation instead of a shared history/culture (well, except WASP culture, which is on the wane).

Russian descent appears not to be a necessary condition for a Ukrainian to "root for the home team". (Neither is it sufficient. But it does correlate.)