I was thinking about Bobby and I actually like the subplot they're taking with Bobby.
Like in many supernatural shows, where people are brought back in some manner after they die. Supernatural has run the risk of devaluing the deaths of characters. I mean, half the time when a main character does die, a lot of us just wait for them to bring him or her back in some way. Either as a ghost, or through resurrection or something other like that.
The interesting thing that SPN does with death is that they've made it clear that, yes, there is life after death, but it's far from perfect. Either you go to hell and torment with the chance of becoming a demon. Or you go to heaven, with is only slightly better since it's lonely and it's basically a constant rerun of life's best moments. Or... and this is even worse, you're a ghost and risk becoming a threat to all around you.
By showing what happens to Bobby, they've now made it clear that yes, if one of the boys dies and stays around as a ghost, that this won't be a happy ending. Making us fear it. Because if even someone as strong as Bobby is susceptible to becoming a vengeful spirit, then the boy's chances of going the same way only increase.
It also shows them sticking to their mythology which can only be a good thing, reminding us that all those other vengeful spirits the boys face might have started out as nice people as well. Until they lost control. And that the boys really are doing those spirits a favor by helping them move on.
Secondly... Kevin... I think I'm more worried for his sake now, that we found out he's still alive, than I was last week, when I thought the Leviathan had just killed him.
Now I'm convinced he's still alive, his mother on the other hand, her I'm not so sure of. (as someone else said, the risk of letting her go would be stupid,)
What better way to control Kevin, than to have his mother replaced by a Leviathan?
So why not just kill Kevin? Simple, what if he mistranslated the word, or if they find something else that needs translating. You don't kill a potentially useful tool, unless you're pretty damn sure his usefulness has run out. And since he's the only one that can translate the word, well, like the alpha allowing Dean and Sam to live until after he checked out what they were saying, the Levis have to keep Kevin alive until they're sure he didn't intentionally lead them the wrong way.