Detailed Explanation
This item is currently bugged.
Executes are supposed to instantly kill any non-boss enemies. For bosses (and red ghosts) it's supposed to deal 2% of their max HP to them when it procs. However for joe's dagger this does not work in the current patch. Each execute usually just deals the same amount of damage as the projectile that proc'd it.
Everytime an execute happens, you do still get the +1% damage on every proc though, which is why the item still makes sense to run.
Note that under normal conditions you can MAX get 10 copies of Joe's Dagger in-game.
You can however get more anyway with a lucky shady guy. The items the shady guys sells are determined as soon as you enter the stage, and they can have a Joe's Dagger. If you get a 10th dagger from chests, the shady guys that spawned in will still be able to sell you an extra copy, although you theoretically are over the limit.
You may also be able to get another one with microwave shenniangens (although if you try to microwave one after getting 10, you will get an error message). But you are able to leave one in the microwave and come back later after getting more than 10 to pick it up.