I think it matters more how you portray their thoughts and actions than I can really get from the above bit... even if I was really at all familiar with said series/fic. For instance, if said 'Complete Monster' was on the ground killing people in Colony Four with swords because he likes to hear screams and lick blood off his blades its one thing. If he participates melee because he wants to not have his troops do things he is unwilling to do its another ball of wax. Regardless of if he enjoys it or not. OTOH, if he leads from the front with sword because he thinks his ground commanders are toeing the line into 'Too Stupid to Live' territory and doesn't want to be the idiot that lost to a colony of unarmed civilians its another thing altogether.
In other words, this is the kind of thing that requires you to do establishing scenes of their reasoning and reinforce it through out the fic. You have to also do it in a manner that makes it seem an organic part of the story and not as if the antagonists are reading your plans for them off a clip board making constant check marks and flipping pages where appropriate.
In short, you must be paranoid about the writing rule :'Show, don't Tell'.
At worst you want the reader thinking, "Eh sure, why not." as a goal your looking to for the reader not actually noticing these scenes exist for the purpose of establishing antagonist motive.
In other words, this is the kind of thing that requires you to do establishing scenes of their reasoning and reinforce it through out the fic. You have to also do it in a manner that makes it seem an organic part of the story and not as if the antagonists are reading your plans for them off a clip board making constant check marks and flipping pages where appropriate.
In short, you must be paranoid about the writing rule :'Show, don't Tell'.
At worst you want the reader thinking, "Eh sure, why not." as a goal your looking to for the reader not actually noticing these scenes exist for the purpose of establishing antagonist motive.