Question

Does poison becomes more harmful or less harmful after it expires?

Answer 1

First off, "expiration dates" are not some kind of deadline, where it was fine yesterday and worthless tomorrow. They're the time in which the manufacturer claims that the product will be up to their quality standards. It makes no claims after that.

"Not up to quality" for a poison could mean anything. It could be more toxic; it could be less. It could be differently toxic, so that it's no longer as effective against its target but could cause severe harm to other species. It only means that one chemical may have started to degrade, and the end product of that degradation could be very different. 

So don't get too hung up on the category "poison". Poison are just chemicals, and time affects them in the same way other chemicals change over time. And don't get too hung up on expiration dates, either: they're just guesses given the wide range of storage conditions.