The kill effect only occurs when you trigger the ability, which is a reaction to losing a challenge. It does not impose a terminal condition like Dracarys.
Venom can slowly eat through duplicates and saves. If attached character is saved, you can trigger Venom next time you lose a challenge. The tokens are not removed when you attempt the kill.
- Venom tokens are placed on the attachment, not the character. So multiple Venoms will not work faster.