The writers for "Supernatural" love to use the monsters as a metaphor for human frailty, particularly when something related to the Brothers Winchester can be illuminated in the process. The action in the episode is all about the "monster of the week", but the subtext is all about Sam and his current path. It's a familiar situation for devoted fans of the show.
Generally speaking, the idea harkens back to episodes like "Bloodlust", where Sam and Dean argued over the possibility that something demonic in origin might manage to live without causing harm. It's the old nature vs. nurture argument with an Antichrist spin. The monster in question in this episode (don't ask me to spell the name!) had the potential, however remote, of controlling its hunger and remaining within society.
Even before learning the full extent of his abilities (and we may not have seen that yet), Sam would champion the more humanitarian approach to hunting. This makes a certain amount of sense; Sam was outside the Hunter world for a while, so he gained a bit more perspective. Dean has normally taken the hardline approach, with the primary exception being Sam.
Now Sam has even more reason to want to believe that someone with demonic blood can control the temptations and instincts that come with that dark territory. He wants to believe that the abilities granted by Azazel can be applied to save people and fight demons. And there is the distinct possibility that Sam could trend that fine line on the basis of his own conscience.
Two things get in the way of that assumption, however, and they are Very Big Things. First, there's the small matter of Ruby. She may be possessing a horrible actress at the moment, one that makes me question Sam's taste in women, but she's a demon with an agenda. And she's never hidden that fact, either. Her influence on Sam is hardly something to be taken lightly. Besides, true evil never seems to be quite so bad at the beginning; in fact, it often seems ideal and "too good to be true".
Second, thereÃ¢â¬â¢s the Travis factor. Given a choice between right and wrong, despite much internal pressure to do the wrong thing, simple humanity and conscience can override instinct. Applying too much external pressure can disrupt that delicate balance in the worst possible way. Jack was under enormous pressure to give in to his hunger, but he was fighting himself off the ledge. Travis came along and disrupted the balance, driving Jack to follow his instinct.
This lends credence to the insight offered in some of the comments for the previous review. Castiel and the angels could stop Sam if they wanted to do so, but it would sacrifice the opportunity to figure out what Azazel was trying to achieve. Also, should Sam survive the initial attempts (which I suppose the Antichrist Superstar could do), the sudden and harsh opposition from an external source could drive him completely to the dark side.
Dean, on the other hand, represents the perfect tool for defusing Sam and uncovering Azazel's plan. Dean circumvents, for the most part, the "external trigger" problem. Sam may fight with his brother, but he'll also listen to Dean more readily than anyone else. It's not a sure thing, of course, because Sam still offers resistance. But clearly it represents the best chance for success from Castiel's view (assuming heÃ¢â¬â¢s being honest in his intentions).
As for what Sam is meant to do, it's hard to tell. According to Azazel and Ruby, Sam was meant to lead a demonic horde to some unknown purpose. Lilith stands in opposition to that; could Lilith and Azazel have been at odds for control of Hell, or the forces necessary to release Lucifer and become his second-in-command? Sam's powers seem uniquely designed to protect him from certain demonic attacks and assert control through the ability to punish. I think the fact that the hosts are saved from death in the process is a mere side effect that Ruby is using as a pr