Johnny McKee
Johnny McKee is one of the more terrible killers we've seen on the show so far. He's also one of the best explored characters. He poisons with nightshade, and another really complicated concoction that I won't even attempt to remember. His targets are people he views as bullies, but his victims are pretty much everyone who has the bad luck of coming into contact with the poison. He has a pretty interesting back story, and I like that his reason for killing him getting revenge for what was done to him and not just him being a crazy guy.
Like I've been saying for a few weeks I'm not totally cool with them giving every single bad guy a reason to kill and be a baddie but I think it worked here. I was legitimately scared of McKee throughout the episode because he didn't seem crazy and I was happy that they confirmed that. I mean, he was crazy in that he's a homicidal maniac, but he is in full control of his facilities and knows exactly what he's doing. They did remark at one point that he didn't see it as murder because the people he killed were not good people, and that is crazy. But again, killing for revenge is a reason and most of the killers we've seen on the show haven't had a reason at all for killing innocents.