Poisoning a soda can with bleach seems like the dumbest, most time consuming, and obvious way of poisoning anyone ever. Workplace politics can get wacky at times.
If I had to guess, I would assume that, like most places like that, employees get free soda while they are working. I am also assuming that these are fountain drinks (paper cup, fill soda at a machine). I would not be surprised if someone (shit it may have even been tevin) cleaned that machine and never rinsed it out fully. The hoses in those things get absolutely disgusting if they aren't cleaned. Like someone said before me, it's more likely that this happened out of stupidity than pure malice. Unfortunately, campus police are notoriously exceptional.