In Black House Parkus/Speedy states that the Crimson King is "pent in a cell at the top of the Tower, but he has another manifestation, every bit as real, and this lives in Can-tah Abbalah - the Court of the Crimson King." Like Jack says, the Red King is in "Two places at once."
The second manifestation is Los' the Red King, the creature Roland faced at the end of his quest. We know this because Los' was the one who dwelt in Le Casse Roi Russe, the court and castle of the King. As Jack says in Black House, the Crimson King "is presumably hanging out in his court (the part of him that isn't imprisoned in Speedys Dark Tower that is)."
We learn of this manifestations origin from the comics....that, basically, it was the result of a Great Old One (the Crimson Queen) raping Arthur Eld.
This raises some questions. Firstly, how does the Crimson King acquire a second manifstation? I believe Insomnia holds the answer....it states that:
"The Crimson King jumps from body to body and generation to generation like a kid using stepping stones to cross a brook."
This implies that the Red King hijacked the body of Arthur Elds and the Crimson Queens offspring, deeming it a useful body to have. This means that Los' the Crimson King is mearly an avatar. But who - and what - is the true Crimson King imprisioned in the Tower since the beginning of creation?
Well, the comics state that "Gan was not the only demiurge to arise from that primordial magical soup." They go on to say:
"...that which came after the bright light of Gan dragged like a heavy shadow. What bubbled up from the depths of the Prim with a great burping stench of decay was the force of the Outer Dark,"
If Gan is the White and the Purpose then the true Crimson King is the Outer Dark and the Random. This fits in with Rando Thoughtful, the Crimson Kings Minister of State, saying that "Big Red always was Gans crazy side." When Stephen King talks about how the two big cosmics (Gan and the Crimson King) were fighting over him, he mentions a name a few times that isn't repeated anywhere else. Heres an example of this:
"I turned aside from Dis, I should be able to turn aside from Gan as well."
So Dis seems to be the name of the true Crimson King pent in the Tower, the being whos Outer Dark and Random influence filters down from his place in the Tower throughout all the other levels of creation. And that, I feel, is why the Crimson King wants to destroy the Tower....because his influence is limited so long as reality exists. In Black House Parkus states that "the King has been trying to destroy the Tower and set himself free for time out of mind. Forever, mayhap."
Black House also goes on to say:
"If he - or it - destroys the Tower, wont that defeat his purpose? Won't he destroy his physical being in the process?"
"Just the opposite: he'll set it free to wander what will then be chaos...."
So, basically, the Crimson King wants to destroy the Tower so that his true self can rule the Discordia without any limitations to his power.
The Dark Tower 7 mentions the Crimson Kings pact with a higher being. I'm refering to Mia when she says that Los' has been "promised his own kingdom, where he'll rule forever, tasting his own special pleasures." She goes on to say that she doesn't know who made such a promise, that maybe "this is only what he has promised himself."
To me, this sounds like the true tower pent Crimson King (Dis) promising things to the weaker second manifestation of the Crimson King (Los'). In essence, he has promised himself these things.