I did a google search on this, but got on-topic-results only on fairly old threads and I don't know if what I read was patched/tweaked since then.
The wiki states Poison does damage mobs that are getting healed; some of the old threads state this is false and that it only prevents healing or that it only does weaken the healing effect.
I wasn't paying much attention the times I, in good faith, poisoned everything around a mender, but I can't really remember anything ever dying from it.
Which is it now?
Also the monsters attack is nearly halved, but how much (in approx. %) is its defense lowered (or rather my attack affected) by strong poison?
Yeah, the wiki status pages aren't very good. Improving them has been a long-term project of mine, but I haven't had much time for the SK wiki in the past two years. Meanwhile, the single best resource for details such as percentages is this thread and its attached spreadsheet. My understanding is:
* A poisoned knight or monster cannot be healed. But it does not take damage from healing. (Whoever said that was probably thinking of cursed mender/silkwing healing.)
* A poisoned knight or monster has a defense debuff. That is, its armor is decreased, so that it loses more health from attacks.
* A poisoned knight or monster has an offense debuff, so it inflicts less damage.