According to Russia, this definition have in several instances included regular Ukrainian troops, who according to the Ukranian government, is fighting terrorists/insurgents that are targeting Ukranian civilians on Ukrainian territory. See how this gets messy quickly, when either side have completely different versions of events?

The "we need to protect our civilians (even if they live in a different country, were born there, and have never been our citizens) against individuals who are targeting them" argument is one of history's oldest excuses for going to war. Sometimes it may be justified, but it also very often conveniently ignore that the "targeted civilians" may themselves have been insurgents or terrorists or infiltrators who were legitimate targets.

It's a popular excuse whether or not it is true, because assuming your attacks go well, it gets very hard to find out the truth when everyone involved has been conveniently killed.