For this achievement, you need to find the punch bowls, kegs or even poison in the brief cases. An example would be the first level, BBQ Party, where there is sometimes a punch bowl on the table in the kitchen. All you have to do then is walk up to it and hit
to poison the food. People will then walk up to the poisoned food and you will know who is poisoned by the green bubbles above their heads. Or if you have a brief case with poison in it, simply walk up to someone at the party and press
to poison them. The achievement will pop after the 100th person has died.