I agree it could be introduced to “check it out”, but if you had a poison that could be introduced into water, would you test it by polluting your own water supply?
Would make more sense to have al ittle bit of geographical separation to do it.
Depends on your frame of mind. A mall shooter expects suicide by cop... Who is to say about a jilted lover or someone wanting to make a political statement. There are a lot of crazies out there. I would not exclude anyone from any action.