I'm actually surprised by how little attention this song has gotten among SPN fans. I once saw a Sam-focused gif set on tumblr that used some lyrics from this song, but otherwise…not so much. Which is surprising because not only is the song absolutely perfect for Sam, but the band is wildly popular.
For those of you who are not familiar with the song, a Youtube link:
And here are the lyrics: http://www.azlyrics.com/lyrics/imaginedragons/monster.html
Almost every single one of those lyrics is relevant to Sam's story. I'm going to give my interpretation here.
"Ever since I could remember,
Everything inside of me,
Just wanted to fit in"
This fits nicely with young Sam's desire to be normal - but the lines "ever since I could remember / everything inside of me" also work as an echo of the retcon in The Great Escapist that Sam felt impure and unclean even as a very small child. For me, the verse is both about Sam's supposed lifelong "impurity" and his lifelong desire to not be that way, to just be normal.
"I was never one for pretenders
Everything I tried to be
Just wouldn't settle in"
In Skin, Sam admits to Dean, "You know, the truth is, even at Stanford, deep down, I never really fit in." Sam's attempts at be normal don't work out - and S4, he rejects the very concept: "Yeah, but the normal rules don't really apply to us, do they?[...]I know you want to think of yourself as Joe the Plumber, Dean, but you're not. Neither am I. The sooner you accept that, the better off you're gonna be."
"I get the feeling just because
Everything I touch isn't dark enough
That this problem lies in me"
Basic summary of Sam's opinion of his freakishness and the demon blood he was fed as a baby. "I've got demon blood in me, Dean! This disease pumping through my veins, and I can't ever rip it out or scrub it clean! I'm a whole new level of freak!"
"Can I clear my conscience
If I'm different from the rest?"
This verse makes me think of S2 Sam. In House of the Holy, we see that Sam, despite not having done anything yet, feels so strongly that he needs redemption that he is susceptible to the influence of the ghost of week, who motivates people who want redemption to kill.
"Do I have to run and hide?"
When Sam first finds out from Dean about their father's last words, Dean begs him to lay low. But instead of hiding, Sam disappears - and starts tracking down information about his dark destiny.
"I never said that I want this
This burden came to me
And it's laid its home inside"
More of Sam's attitude towards the demon blood in a nutshell - and about the responsibility to stop the apocalypse that stems from it. As Ruby says, he's the only one who can kill Lilith.
"If I told you what I was
Would you turn your back on me?
And if I seemed dangerous
Would you be scared?"
I think "you" in this set of lyrics is very much Dean. Dean is appalled by Sam's actions, using his powers, drinking blood ("If I didn't you know... I would wanna hunt you."). Sam's hallucination of Dean's reaction is even worse ("You were always a monster. And you only feel right when you're sucking down more poison and more evil.[...]And I tried so hard to pretend that we were brothers. That you weren't one of the filthy things that we hunt. But we're not even the same species. You're nothing to me.").
But Dean's reaction isn't only rejecting. He's also scared. If I recall right, Dean flinches when bloody-mouthed Sam moves to stop the demon behind Dean in The Rapture. The choking attempt in When the Levee Breaks was bad enough that Dean had some difficulty drawing breath to talk - Sam could have killed him. Sam's hallucination of Mary describes Dean as "terrified". In Dean's own hallucination, yellow-eyed Sam tells him, "No one's possessing me, Dean. This is what I'm going to become. This is what I want to become. There's nothing you can do about it."…and tries to choke him.
"I'm only a man with a candle to guide me
Candle = Ruby. She's the one guiding Sam, only a candle in the dark because she gives him limited information.
"I'm taking a stand to escape what's inside me"
This line alone would convince me the song was about Sam. As he says in Metamorphosis, "I'm just trying to take this - this curse... and make something good out of it. Because I have to." It's S4 Sam's motivation in one line. Sam is trying to escape the curse that Azazel laid on him and his family. He's taking a stand by embracing what he can't escape, the demon blood, and attempting to use it to regain power over his life and the demons who have tried to take it away.
"A monster! A monster! I've turned into a monster!
A monster! A monster! And it keeps getting stronger!"
"I can feel it inside me, Ruby. I've changed... for good. And there's no going back now."