It'd be hilarious if the monkey was just playing on their paranoia, and didn't actually poison the water.
Although, that's more of a sadist move than a slasher move, so I doubt that's how it's gonna go.
Hang on a second but they're a pharmaceutical company, shouldn't there be some hope to finding out the poison in the water and an antidote? Well maybe not, cuz the equipment is not available to examine