Typically, for BTU we are talking about indiscriminate use of firepower and indifference of of collateral damage- I burned down the house but by God the spider is dead. And sometimes its what ends up as collateral damage, in one of the FCCW novels, the POV character who is a battalion commander for the New Syrtis Fusiliers gets upset because the attacking Katherine loyalists start shattering the ice art of the Hasek Memorial . . . and to survive he has to fire back.
Then we got the St Ives militia tanker who was not sure if his infernos were the ones lighting off some wooden village on Nashur. His lance leader was blaming a Capellan for such a war crime-esque action, it rocked him that it was someone from his side. I also seem to remember a FRR unit that swore death to a Invasion Wolf cluster commander (Crusader, what a surprise) for his actions during the invasion of the world they were defending which caused a lot of collateral damage the Clan warrior was indifferent in creating. The FRR unit eventually launched a unsanctioned raid to smash the Wolf cluster in the 60s- lol, the cluster had been disbanded.
War crimes can be more than just killing or destroying indiscriminately . . . a commander could order his medical support to use prisoners as non-voluntary organ/blood donors if medical support was low. The prisoners could survive, though be mutilated or with long lasting health problems . . . so they are not dead, but its extremely grisly and a abuse of prisoners. While normally a human rights violation and could be considered a crime against humanity, because it was done as part of a military operation makes it a war crime.
Making prisoners build defensive works or in 'defense industries' is IIRC a war crime, b/c they are not supposed to be compelled to labor against their own countries . . . but its a minor one, can easily be worked around, and is fuzzy thinking . . . if a nation does not have to commit available labor to harvesting crops, then they can shift that labor to say building bombs. The product, food, would even find its way into feeding troops or the labor that builds weapons . . . or does not require food rations to be shifted to feed prisoners.
But for a BT commander we are talking about killing civilians or prisoners.
Btw, being made a prisoner is not a right and under certain conditions a military commander will be compelled not to take prisoners. As mentioned earlier, a lot of this is a modern conceit and we can get into more of the theory of modern use in PMs.