The roommates kill 10 people in all and begin to fight amongst themselves over if they're doing the right thing.
Eventually, they invite the famous politician, Dr. Arbuthnot (Ron Perlman), who the five roommates share different opinions about, over for dinner. They go into the kitchen to argue about whether he should die or not (it comes to a 1-4 vote with Luke the only person that wants to kill him, condemning him as "Hitler"), and while they fight, Arbuthnot snoops around and puts together enough evidence to realize that the roommates have killed those 10 people and the missing sheriff. He figures out that the blue bottle of wine is poisoned (he can smell the large amount of arsenic Marc poured in it), and pours some of it for all five of the roommates when they aren't looking. He then proposes a toast, and they all drink the wine.
The movie ends with a painting of the dinner table, the five roommates dead around it with the politician standing alive with the blue bottle of wine beside him, with Arbuthnot speaking about how he may be running for President.
It isn't confirmed, but it is assumed that Arbuthnot poisoned all of the roommates, killing them with their own wine.
Thanks Salvatore D!