Answer: Tomatoes
The group kills people with radical ideas, with the idea that it is better for humanity if these people are dead. They use the hypothetical question of, if you could travel back in time to say 1916, and you're in a bar in Austria talking to an art student who happens to be called Adolf, would you kill him, given the knowledge of what he will do in the future? But say you hadn't travelled back in time, you didn't know what was going to happen, you just knew his ideas weren't right, what would you do then?
This is how they justify the murders, they then plant tomatoes to try and cover the fact the garden has been dug up.