I'm not a PM but if I were to answer I would ask about the user base (just to show that I am user focused, I want to get to killing right away). Then I'll think of cheapest solutions. Bombs,guns etc are operationally expensive. So I'll go with poisoning. Easiest method will be gas.
As for the robots, putting AI in every robot is again expensive. So instead I would prefer a fleet of robots, controller by an AI. Decision making is centralised and it is easier to stop in case of any bugs.
But gas propagation is also hard to control, so we might want to use data analytics to figure out where and when the majority of our targets will be together.
Example, if they all are same company, are they of same educational institution etc and try to optimise it.
Each robot will keep count of the kills, misses etc and we will use it as feedback to improve our next versions. Monitoring and feedback is essential
(I have no idea what PM does, I just wanted to try making cheap killer robots)