Haco happened to give Cane a poison, something that he said Iris wanted him to make out of impulse, which later on, his mate told him about the grievances she felt and didn't really mean it. Especially when Oliver had died.
However, because the poison had been made, it would be a waste if Cane didn't use it for his own benefit, especially with the uniqueness of the poison.
It was a slow acting poison that could be fatal, only if it was triggered. To put it simple, like an allergy, you had it, but it wouldn't be a trouble if you didn't come in contact with the source of your allergies.
This poison was the same like that.
Knowing this unique poison that Haco created could benefit him, Cane asked the sorcerer to make more, which was not easy with his current condition, but in the end, Haco managed to do it.
