depends the poison :D

Likely less. The expiration date on pharmaceuticals is when their potency has gone down because they have degraded to more simple chemicals. Unless the decomposition of the poison produced more dangerous compounds, it should be less toxic than before.

It will depend on the poison. It most likely won't have quite the same desired effect, but that does not mean it's less dangerous. A quick acting poison might turn into a slower & more painful one. But most likely, the potency will go down.

I know a spitting cobra is much less lethal and dangerous when it's dead... Because it can't spit.

less effective, usually

Neither, or less. More is unlikely.

You buy the poison to kill someone ASAP. It never gets to expire and if it did something must have gone terribly wrong.

...Best before: <insert your death date>...

Pretty sure I'm past my expire date, didn't get more dangerous.. yet..

A number of poisons, such as hemlock, degrade over time, so I'm pretty sure they're usually less dangerous.

If it would become more potent after it's expiration date, it wouldn't have one.

But if it becomes more poisonous, then would it have really expired?

It doesn't really matter since I very much doubt the victim would be able to complain.

It changes its effect, sometimes lose it sometimes became something else more dungerous

Too far past its expiration date, poison becomes vintage.

In any case, I think science can answer this. It will result in a number of casualties, though.