Specific types of frustration or seeing friends die in certain ways can do things to the human mind.
Snipers didn't carpet bomb residential areas, but they often didn't have a very nice fate once captured...
I don't want to reduce real war to a video game, but I think the fundamentals of frustrations over roles in war can even be seen here already some times. When you play a vanilla battlefield game with casual gamers, have you never seen frustrations over things like "pussy tankers bla bla bla". It's basically someone who was faced with a tank, and he didn't have the weapons to balance the fight out (no AT weapons for example). He is frustrated by his powerlessness and feels like it's an unfair fight where the other one is just exploiting a powergap not necesarilly related to "courage" or "skill", and once the power balance changes (tank gets deactivated or surrenders or whatever), it isn't unlikely that the frustrations will turn into bad behavior towards the "not so tough now huh" person, where the frustrated person might see it as "just" to "compensate" for the earlier "imbalance".
You can also think here of that Jordanian pilot that was captured by ISIS. He wasn't the "carpet bomb civilian areas" kind of pilot, but ISIS has the frustration of constant jet bombardments on their positions, and they don't have the AA quality to shoot those jets down. When they then get in the position where they catch one of these pilots (by them considered as "cowards" as the pilots know that the chances of getting shot down are very very low, while the chances of their bombs hitting are high), their frustrations may lead to an extra cruel treatment compared to other POWs.
