Even though "Set Me Free" by Velvet Revolver is associated with the Hulk film from 2003, the lyrics make me think of The Immortal Hulk. Let me break down my thoughts:
You operate and motivate on synthetic fuel
This one might be a reach, but Bruce's "synthetic fuel" may be his telling himself that he can use the Hulk to change the world for the better.
In his conversation with Amadeus, Bruce admits that he doesn't believe in heroes, that he doesn't see *himself* as a hero, that *he* can't build a better world. His plan is to aim the Hulk at those who are slowly burning the world down for profit, and let the task of building something better "fall to other people" in his words. It's worth wondering if Bruce really believes the new world will be better the one he's setting out to end. If he doesn't have what it takes to build that world, then who does?
Perhaps he really does believe in what he's doing, I'm not ruling that out by any means. But at the same time, I think on some level he wants a sense of belonging and purpose. If he can't be a hero, maybe he can find some meaning in being a Hulk.
You're mother nature and an atom bomb
This makes me think of the dual nature of the Hulk. Bruce Banner is a man, and a monster. Sometimes a savior, sometimes a destroyer. The Devil Hulk wants to end the world in order to save humanity. Gamma radiation is both science and magic. The ones above and below creation are two faces of the same entity.
The reference to "mother nature" could also tie into Devil Hulk fighting the forces causing climate change.
As long as you're kept full of pretty bodies
Your little secret will be safe with me
This line I'm admittedly unsure about.
The first thing it made me think of was actually Dr. Frye, creating many dead bodies both directly and through his son Del, all because he wanted to keep his own wrongdoing secret. But the rest of my thoughts on this song are about Bruce Banner and the One Below All, not Dr. Frye. Perhaps Dr. Frye's actions could be seen as a reflection of the human world that Bruce wants to end, allowing many to suffer and die because those with power are unwilling to confront the source of the problem?
Or maybe it's a reference to Bruce containing multitudes, being plural, being the ideal vessel of the One Below All (and Devil Hulk reforming around Dr. Clive that one time)?
The term "pretty bodies" also makes me think of the One Below All using the forms of others, like Rick Jones, Thaddeus Ross, Jackie's dad, but those forms being hollow shells of the people they represent.
Around again, insane again
It comes again and sets me free
A reference to the cyclical nature of the Hulk's story, and the stories of comic book heroes' stories in general, always snapping back to their status quo. Though, admittedly, that sounds like quite the opposite of being set free, at least in the long term.
So set me free, set me free
'Cause I think you need my soul
Set me free, set me free
This line especially makes me think of The Immortal Hulk, specifically those possessed by the One Below All, who needs to wear the souls of others as masks, as it does to Brian Banner, Samuel Sterns, and Bruce Banner/the Hulk in the far future.
Though the one being set free isn't Bruce Banner, but the Hulks, who were always a part of Bruce, but given new power and influence by the Gamma bomb that also opened the Green Door. In a way, Bruce set his own alters free and set the One Below All free with the same act.
You're kept alive and polarized with one thing in mind
Metabolize everything that you see
Another reference to the One Below All, this time it's desire to destroy everything and leave itself as the lone presence in the cosmos, a desire reflected in the Hulk's tendency to rampage against those who won't be left alone.
This could also be taken as a reference to the human world, consuming resources with little regard for sustainability.
But now and then or a little later
Now I'm gonna take you down with me
This makes me think of the future where the One Below All possesses Bruce, and how it's framed as inevitable, not even undone by Farys's tiding fly.
Alternatively, this could also be in reference to Bruce resolving to tear down the human world, even if it means burning bridges with the superhero community and being branded as a terrorist.
So take me down
Take me, down, down, down, down
Take me down, take me down
The Immortal Hulk constantly makes use of "the Below-Place", "the One Below All", "as above, so below". Could also be taken as a reference to Bruce Banner's death wish.