Suppose a player uses the Hack and Slash move against a monster, and he obtains a partial success (7-9). He rolls the damage and he is able to kill the monster, can I as a DM use the Deal Damage from the monster even though he was killed?
It could mean that the monster inflicted some damage to the player before it was killed. Does it make sense?
That makes perfect sense. Remember though: Hack and Slash is not just that one strike, it's the synopsis of a fight. But if you want a particular moment to stand out, because it's truly epic (like a difficult boss finally going down, for instance) you could use the move just for this single strike.
Maybe the player had to get close to the monster to deliver the killing blow, and the monster got the chance to claw and bite while it was being gutted? Or the player lost control of his/her weapon while striking, dislocating a shoulder in the progress? Or the dead monster fell on top of the player, its spiny exoskeleton piercing the player's leather clothing? Or the monster coughed up a ball of bile with its last breath and the acid burned the player's face?
It makes perfect sense. But remember to keep the fiction flowing afterward, don't just deal damage: Is the player now disabled in some way? Is he pinned or blinded? And what's next?
Yes, that perfectly makes sense. It helps to see H&S as a sequence of combat not just a single hit.
I use this situation to help my players being imaginative. For example I ask them "okay, you kill the monster but he/she also injures you. How do you imagine these last actions?" They generally come up with a series of hits and parries where their PCs get slightly hurt before striking with the final blow on the enemies. These kind of tricks are really helpful to make D&D players moving towards a more narrative kind of game, such as DW.
Another available option depends on the number and type of creatures that are actually participating in the fight. If there are other creatures like the one that has just been killed around, I make these ones dealing damage on the PC, e.g. "Your sword blows the final strike, chopping the head of the bandit off. He goes down, but another bandit seizes the moment and shoots an arrow at you, dealing 1d6 damage". Just remember that, to be compliant to the rules, both creatures must have the same damage.
