The number cited, which is around 14,000 represents the total number of people killed in the fighting in the Donbass region from 2014 until the cease fire there ended with the recent Russian invasion.
You can look at the details of the estimate on Wikipedia, based on sources like the UN and the Ukrainian government.
The majority of the people killed were soldiers fighting for the Ukrainian government, or the LNR/DNR militias, or the Russians fighting with the LNR/DNR militias.
According to the reports published by Wikipedia, about 3,404 civilians in the Ukraine and in the separatist controlled regions of Ukraine died in the fighting.
While some war crimes may have occurred, like the Russian shelling of a pre-agreed evacuation of unarmed Ukrainian soldiers, civilians on both sides primarily were killed by shelling of the villages and towns along the front lines by both sides.
The talking point we keep seeing here on Free Republic is that 14,000 citizens were killed by Ukrainian shelling which is simply a lie. The 14,000 refers to the total number of people killed in the fighting, of which around half were killed by the LNR/DNR/Russian forces firing upon Ukrainians that were not in the breakaway provinces. And of the 14,000 more than 10,000 were serving in military or armed paramilitary groups.
Trying to blame one side in a war and in particular the side which did not start the war for all of the deaths that resulted from the war is just a cheap propaganda technique, and a lie.
To answer your specific question if the killing of one civilian in a war is a war crime the answer is no, not if they were killed accidentally during combat operations.
Which have been edited to support the official "Ukraine Good, Russia Evil" Narrative.
Take your clown show down the road, free.