how long it will be until someone dies of allergens slipped deliberately into their food as an act of retaliation (if it hasn't already happened. Feel free to write about this idea).

It has already happened. It happened at a school in the US a couple of years ago. The kids who did it didn't realize that the allergen in question was serious enough to be fatal, and were in fact completely horrified to find out the hard way, but are (quite correctly) being held responsible for their actions.

I've seen some other examples of this as well. There was a horrific confrontation last year between members of PETA and a biker gang that PETA was trying to picket.