If Ukraine wins. Most civilans living in Crimea and in the occupied territories, as ukraines plan decrees, have to leave the country. If they somehow still only have the Ukrainian citizenship they are barred from any job that has any ties to the goverment.

Also anyone with a goverment position who helped, so any policemen, doctor, teacher, etc will probability be judged as a collaborator and hence a traitor.

So imagne the regular Ukrainian Army moves in. With some nationalistic hatred that exists. You will have armed men while everyone around em is an enemy, and there is no one to stop you, some people will resort to self justice. Hence the metaphor of putting civilans on the front line.