Usually, not always.
I take it that you've never had the experience of killing anyone that you've pursued for some time. Never had the opportunity of catching up with and getting the drop on members of a regiment that left one of your youngest Marines skinned alive and begging for death....where you would find him.
Let me assure you - sometimes, simply killing is NOT enough. The need to MORE THAN KILL is to leave a message for those that will discover or recover the corpses - and to drive deep into their nightmares - the knowledge that they are being pursued by demons that mean to kill them and worse than kill them - deprive them of any and all honor or a life after death in their belief system.
Uncivilized, perhaps.. Brutal, to a degree... Necessary, who knows.. Satisfying -- a little....
Stupid, as you believe?
Not by a long shot...Whatever it takes to destroy, disturb, demoralize and defeat the enemy that means to destroy you - is not stupid.....it is the path to survival.
Semper Fi