At least, not if Dean Winchester has been possessed by the Mark of Cain, killed, and then brought back as a murder-craving demon. See for yourself, in this clip from last night's Supernatural. Spoilers ahead...
A little context for the above clip. Basically, Dean is still just in the Sheryl Crow school of demonhood — all he wanna do is have some fun. But Crowley wants Dean to be his new bestie, and also his new enforcer. And Crowley also knows that Dean's bloodlust will get more and more uncontrollable, so it's in everybody's best interest to send out Dean to kill people Crowley wants killed. Kills two birds, etc etc.
In this particular instance, a guy caught his wife cheating on him, so he demanded a divorce — but his wife wants half of his hard-earned money. So he sells his soul to Crowley in exchange for his wife ending up dead. There are a billion ways Crowley could make that happen discreetly, ranging from magic to having demons possess one of the wife's friends — but instead, he decides to send the homicidal maniac with the bone blade to do it. And then Dean sees the husband spying on the wife in his car... and you can see the rest above.
This is one of two excellently bizarre moments in last night's "Reichenbach Falls." The other being the bit where Castiel (having crashed his truck) is at the house of the truck-repair lady, sleeping on her couch. And Castiel wakes up to see the truck-repair lady's daughter sitting on the couch eating cereal and watching old cartoons. The kid tells Castiel (who never dreams) about one of her own dreams: her snot became a rocket! It flew into space! It destroyed a bunch of stars to make room for more rockets. Castiel can only observe that this sounds like very special snot.
As for the actual plot of the episode... basically, Crowley and Dean "break up" and Crowley sells Dean out to Sam. But meanwhile, Sam is being followed by a psycho named Cole who saw Dean kill his dad back in 2003. Cole thinks that Dean is a monster — and the irony is that he's actually right. He wouldn't have been right if he'd caught up with Dean last year, but now he's spot-on. Dean kicks Cole's ass while making Princess Bride jokes. And then Sam captures Dean with special handcuffs, in all the confusion.
And in the "B" plot, Hannah the angel is worried about Castiel — so much so, that she comes close to making a deal with Metatron to let Metatron out of his cell in Heaven in exchange for what remains of Castiel's Grace. The scene where Castiel shows up and puts a stop to this chicanery is pretty great, and shows why Misha Collins is still the man. Especially love Castiel saying that the thought of Metatron rotting in a cell for all eternity is his "happy place."
Anyway, Dean insists he didn't show mercy on Cole the vengeful psycho, when he spared Cole's life. Because living with the knowledge that his revenge fizzled is the worst thing that could happen to Cole. And what Dean is going to do to Sam won't be mercy, either. And Dean proves that he's gone completely evil — because he doesn't even care about the Impala. Noooooooooooo!