Laius is such an amazingly fast thinker. The rules of the dungeon change the risk calculation quite a bit.
The greatest danger is not dying, but not being revived in the future.
He aimed for the only reasonable goal in that scenario; subduing Thistle and disarming him from the book even at the cost of being mortally poisoned. At worst he may force Thistle to heal him and his party, at best he's able to talk things out with him.
I applaud the writing here!