An example of that:
[
{
"id": "mon_jabberwock",
"type": "MONSTER",
"name": "jabberwock",
"description": "A putrid amalgamation of body parts from humans and other creatures have fused together in this aberration of flesh. The eyes of all the heads dart about rapidly and the mouths form a chorus of groaning screams.",
"default_faction": "jabberwock",
"species": [ "ABERRATION" ],
"volume": "875000 ml",
"weight": 200000,
"hp": 400,
"speed": 140,
"material": [ "flesh" ],
"symbol": "J",
"color": "dark_gray_red",
"aggression": 100,
"morale": 100,
"melee_skill": 10,
"melee_dice": 4,
"melee_dice_sides": 8,
"melee_cut": 3,
"dodge": 3,
"armor_bash": 12,
"armor_cut": 8,
"vision_day": 50,
"vision_night": 3,
"special_attacks": [ [ "FLESH_GOLEM", 5 ] ],
"death_function": [ "JABBERWOCKY" ],
"harvest": "jabberwock",
"flags": [
"SEES",
"HEARS",
"SMELLS",
"STUMBLES",
"WARM",
"BASHES",
"DESTROYS",
"ATTACKMON",
"POISON"
]
},
{
"type": "MONSTER_FACTION",
"name": "jabberwock",
"by_mood": [ "jabberwock" ]
}
]
I think removing the faction flag results in error, so try to make a new faction for your monster 
(have its "by_mood" set the same as the faction ID, that WILL force them to fight each other)
