OOC:
Margaret already has 1 hold from earlier as she got a 10, so no need to roll again. Sorry you waisted a 13. Unless the scene changes and the holds are lost, you don't keep rolling to stockpile holds. You get them, then you hold them until they are lost or spent, and then you can roll for more after that.
Also, moves do exactly what they say they do. You can cause something to happen so it takes 2 harm ignoring armor, but unless a move say it hinders movement then it doesn't. Disease is a bit ambitious, but I will say that is more of a long term thing, not something you will see the effects of right away. When you get a cold, to don't know that right away.
You see a glob of some kind of substance fall from the tree, presumably a blood-like substance as the bullet causes a grave wound. It does seem the monster has a hard outer shell of some kind, perhaps an exoskeleton or carapace, and thus takes 1 less harm than expected.