[RPG] How does Thief’s Poisoner move work

First off, all of edgerunner's answers are great. But I wanted to add some Dungeon World specifics:

Check p.19 and you'll see that 6- isn't "failure" - it's "trouble". The GM will say what happens and the player will mark XP. You are attaching non-DW simulationist ideas to DW mechanics by your supposition that 6- means "failure."

These principles can apply in all sorts of games, and have been used by GMs for years. If the PCs have to climb a fence, they're just going to keep trying until they succeed, right? So even in traditional games, many GMs will read "failed" rolls as a lack of some quality - not fast enough, not quietly enough, not without hurting themselves, etc., instead of just keeping them on the wrong side of the fence.

This is because failure is boring and stops moving the story forward. So you are correct, there is no plain-old failure in DW. It's not in the GM's agenda to make the PCs fail. There is no move for failure.

So the problem isn't that edgerunner's ideas are non-optimal, it's that your concept of what 6- means is wrong and that static failure doesn't exist in Dungeon World.

Expanding on 6-

From the text:

Generally when the players are just looking at you to find out what happens you make a soft move, otherwise you make a hard move.

Somewhere in Apocalypse World itself it says about hard moves:

make as hard and direct a move as you like

Early PbtA games like DW assumed you understood Apocalypse World. And this phrase is often tacitly implied in PbtA games even today.

6- means trouble as I said. The GM is free, on 6-, to make a move as hard as they like. That doesn't mean as hard as you can think of.

AW says:

It’s not the meaner the better, although mean is often good. Best is: make it irrevocable.

So while a 7-9 should substantially give the character what they wanted (they accomplish their intent even if their action created complication), on 6- you are free to deny the intent (the action still has to have consequences beyond "no" though) and in addition make a move as hard and direct and irrevocable as you like.

Climbing a mountain a soft move is "The boulders above you on the rock face begin to wobble as the grappling hook you've tossed up there sets itself. What do you do?"

A harder move is "The boulders have tumbled off the edge of the ledge and after hanging nearly motionless for a tiny instant above you, are now plummeting towards you, gaining speed every moment. What do you do?"

A really hard move is "The boulders are yanked free by your grappling hook and come smashing into you, tearing you from your narrow perch and scattering the contents of your pack into the yawning emptiness beneath. What do you do?"

You can always pick any option you want. It's the GM's job to make them make sense together, by arranging the circumstances believably around them. (Although, as part of the GM's job, they might throw that question back to you using a GM move to make explaining it your job. Either way, it will get explained.)

But, essentially, what choosing “You don’t get into melee with them” from Backstab means is that you escape, at least for the moment. Even if you just stabbed them, choosing to not get into a melee means you stabbed them and then got away. You might now be in a chase, or you might be in an awkward spot, or you might be praying you make a good Defy Danger to blend in with the crowd Assassin's Creed–style, but at least you're not facing live steel right now. You've got some distance and control over the situation. You gave them the slip! You don't stab them and then just stroll off with a “how d'you do” to them as they do nothing; something is there that you leverage to get away from immediate reprisals.

Its meaning might be more easily grasped if you consider what happens if you don't choose it: you are now in melee, and they are going to (try to) hurt you. Being a thief, melee is not where you want to be, and escaping melee is going to be dangerous and possibly painful.

Even if you get a 7–9, it can still make sense to choose to not get stuck in a melee. You snuck up, hoping to do some serious hurt to that fully-plated knight on guard on the wall? Great! Oh, but you roll a partial success, and you can only choose one. Now that you're here, and you will get caught in melee if you don't choose it… maybe what you want to do is just leave, and try something else, right? When the alternative is stabbing them but then being stuck inside the keep, in a fight, just leaving might turn out to be your preference, and you have the option. That's what choosing only “You don’t get into melee with them” can mean: you escape without getting caught. Oh, you might still get noticed, but at least you got away, right?

But isn't it always better to just outright murder them without a move, since you can do that, when an opponent is defenseless?

Not always — a humanoid opponent, maybe, but you don't always face humanoids who are squishy enough for that fiction to make sense (and an auto-kill only happens when the fiction makes sense), even when they're defenseless. You're not going to murder a sleeping dragon! But as a thief, you are assured of a chance to hit that sleeping dragon once, hard, while it's defenseless, and Backstab will trigger in those circumstances.

And their death isn't always the objective anyway — you might be sneaking into the enemy camp to intimidate the General into marching his army away cleanly, instead of killing him and having the army disband into a bunch of brigands that will ravage your homeland. Backstab gives you a solid mechanical assurance of being able to pull off such a non-lethal attack, much better than a bare Defy Danger would in the same situation.