My guess its cockroach poison, seeing as we already know there are cockroaches in their room and Raashi doesn't like them. Poison is poison, wether its rat, cockroach or human. No label says its ok for humans to drink rat poison. 😉
Jokes aside, I truly believe that there is more to the whole deal. He confessed too easily. It would be too simple of a story if ranveer got caught so easily.
Yes, Ranveer going evil isnt' that bad of an idea but how about enraged and out to find his brothers killer, instead of a evil human being?