During the last session, my players sneaked into a camp at night. A lot of enemies (several tens) were resting in their tents. My players managed to set fire to the tents without being caught and so many enemies died in the fire. By considering the regular exp for each dead creature, this means a big step forward (about 4000 exp at level 7). My question is: is it correct to assign regular exp after these "indirect" kills, or is it better to assign only a certain amount for the clever strategy?
You should give the players full experience for the creatures they kill. If using clever tactics results in a decreased reward, you are effectively disincentivising playing tactically and encouraging your players to turn every encounter into a straight-up fight.
However, if you're going to be fair about the rewards of clever strategies, you should be fair about the results, too. Assuming being in a burning tent is roughly equivalent to standing in a patch of burning oil, the creatures in the tents should only have taken 5 damage per round. If it's equivalent to being covered in alchemist's fire, that's a mere 1d4 damage per round. Letting this tactic insta-kill anything stronger than a kobold is being extremely generous to your players. The idea here is not to cheapen your encounters in terms of either the challenge or the reward.
XP in 5e is generally rewarded for two possible reasons.
- You reached an important plot point and the characters go up a level.
- The characters have defeated an encounter and receive XP for that victory.
Defeating an enemy does not necessarily mean that they killed the enemy. You can defeat an encounter by bypassing it completely, or using clever tactics to get indirect kills, or by bringing them down to 0 hp in a battle. All are valid ways of defeating an encounter and all are deserving of the full XP reward.
As GMNoob eluded to, we don't give exp for kills, we give it for getting through the trial - give them exp for the skills they did use.
Give them exp for the actions they took, not the results of those actions. There should never be indirect experience gain. Maybe that's exp for besting a character in combat. Or exp for avoiding combat by sneaking to the safe, picking the lock, and escaping.
Ultimately, its exp for using skills to achieve a goal. Exp for progressing your story.
I think you should set a good amount of XP for this maneuver, but the killing of creatures that a character did not challenge face to face must be lower, even inexistent if you are not aware, like planting a trap and a creature gets killed on it two days later while Im a mile away, maybe if you check the trap later to find the killing you could get some feeling of achievement, hence XP.
The XP of killing a creature represents much the fact that you are matching your skills versus the creature, feeling the danger, learning from your moves, their moves, but something impersonal like "set a clever ambush that defeats an army" should get a good XP, but get the formula 1000 infantry x 10 xp each... 200 cavalry x 20 xp each... its wrong (Army Generals would be overpowered! NERF!)
Think it that way, while setting fire and killing all the army its a clever tactic that must be awarded a good amount of XP, getting into a fight with every single soldier of the army and killing them by your own would be the greatest experience achievement! So full XP! But the probability of surviving on one tactic or the other are so different that you trade experience for survivability.