Some modifiers to this roll. From GURPS Dungeon Fantasy 11: Power-Ups, p. 21, apply a -5 to reaction for each of these:
- Going into the monster's land uninvited. Yes, that means going into a dungeon and it lives there. It also means finding its lair out in the wild.
- Monster is from another dimension (Demons and Elder Things, foremost).
- Monster is truly evil and the delver is truly good (or the other way around).
- Monster is compelled (magical, divine; good old geas and quest spells) to guard an area. Or anything else that would interfere with such a compulsion, frankly.
- Monster is, well, a monster. It eats flesh or life or whatever of the delver's race.
This is a General Reaction Roll. As such, GURPS Social Engineering, p. 21, has these modifiers:
- Roleplaying. Give a small boost for good roleplaying, a penalty for bad.
- Biases. Monsters with Intolerance is the biggest one, likely followed by those with Loner. Some delvers have Social Stigma; most monsters don't care much about that, but the more civilized ones (like elves and orcs) might. Social Regard, which it also mentions, isn't as big a deal. It also mentions Xenophilia, Chummy and Gregarious as ones the monsters/NPCs have that could make a difference.
- Appearance and Behavior. Most of the time, things like Appearance (other than from a racial template) and Voice do not matter to monsters. Charisma works only with other sapients. GURPS Social Engineering gives a few others; GM's call which ones work with non-humans and especially non-sapients.
- Reputation. Again, something that matters to sapients, but not to true savages.
- Skills. Diplomacy and Fast-Talk give their normal +2 bonuses if someone has them at skill 20 and can talk. Otherwise, this is all at GM's call, and again, we're largely talking monsters.
- Social Position. Status doesn't mean too much to black puddings.
Does the monster want blood? If the roll after adjustments is Bad (6) or worse and it really is a savage monster (as opposed to an orc or a dwarf), then yes, it does. For the intelligent ones, it takes a Very Bad (3) roll, though a Bad (4-6) roll triggers a Potential Combat Situation reaction roll.
While we're on the topic, roll a Potential Combat Situation reaction if the party makes a clear move to fight. Modifiers for this from GURPS Social Engineering, p. 68, include Language, Leadership, Outnumbered, Ranged Weapons, Size, Supernatural or Exotic Abilities, and Territoriality. Look 'em up if you want to know the exact modifiers, but they're not big.
There isn't a listed Morale roll in GURPS. There is, however, a mechanic that is pretty darn near: Fright Check. Thus, we can have monsters make Fright Checks when the first one goes down, and when half their group go down. For lone monsters, it's first strike on which it takes damage (at least HP/10 damage, round down, to keep a dragon rolling a Fright Check when it takes a scratch), and when it has lost half its HP. Unlike normal Fright Checks, however, this is a binary result, and doesn't have a roll on the Fright Check table for a failure. The important modifiers are the ones on the character sheet: Combat Paralysis, Combat Reflexes, Cowardice, Daredevil, Fearfulness, and Fearlessness. Don't roll for monsters with IQ 0, or Unfazeable ones, though intelligent Unfazeable monsters (liches, wraiths, vampires) won't fight to the death.
Making this Fright Check means it fights on, though the GM can overrule this for an intelligent monster taking a clear beating. Failing it means it tries to run or, if a critical failure, tries to surrender. Modifiers include those for a Potential Combat Situation reaction roll; the Fright Check takes its spot.