Granted the pride, why did he choose to act on it and rebel instead of resisting it and obeying?
I having problems with your terminology. Act on "it"?
Acted on his pride. Why did he choose to go with his evil desire instead of his good desire? If he only had an evil desire, then he couldn't have done otherwise.
There is no "it". He chooses to turn his back on the good and picks to do the evil, because that is his preference. [emphasis added]
If his preference made him rebel, then how could he have possibly obeyed? If his preference was somehow up to him (or under his control), then begs the question of why he rebelled rather than not.
We call that pride.
Ok, let's be extremely precise: Did Satan feel prideful, then choose to rebel because of the pride? Or did he choose to rebel, which made his character prideful?
If the former, then that means he doesn't have free will. He suddenly feels prideful, which leads him to rebellion. If the latter, that means his behaviour was akin to a roll of the dice, since there's no cause for him doing A over B.
Those who are Predestined are saved from this flaw in the end (we sin 7 times a day at least), those who aren't are left in their natural state.
This doesn't seem to fit with free will either. If God predestines person X, that entails X goes to heaven. If God doesn't predestine person Y, that entails Y goes to hell. Given the inescapable entailment, you have no power to decide your fate. A premise outside your control (God's predestining you or not) is the ultimate decider of whether you get saved.
You guys are going around in circles. "Why did A do B?" "Because he chose to do B." "Why did he choose to do B?" "Because he has free will and made that choice." "Why did he make that choice?" "Because that was his preference." "Why was that his preference?" "Because that is what he chose to do." Etc.
The libertarian is the one who falls into a regress. I'm just putting it on display.
The only rational explanation for why a will chooses evil over good, or good over evil, is because it is desired (e.g. valued) more. If there is no rational explanation for why a will chooses what it does, then humans are simply not rational animals. Their choices are simply random. Which makes Christianity a farce - evil deeds as well as good are at bottom the result of bad luck.
Right. Your will moves based on the strongest, proximate desire apprehended by the intellect -- or it moves randomly. Either way, there is no reason for God to punish, if He or She exists.
But, if desires are chosen by the will, that leads to an infinite regress. The desire chosen is the desire that is desired more. And that desire is the desire that is desired more. And on to infinity.
Well, some desires might be under the control of second-order desires (or third-order desires). That doesn't necessarily lead to an infinite regression, so long as there is a fixed desire somewhere in the chain.
All this forces the conclusion that there is something in humans that 1) is not an act of the will, 2) determines acts of the will by determining desires, and 3) is self-explanatory.
This forces the conclusion that no one is morally responsible for what they do.

1.) "Acts of the will" follow from the will. So the will itself is not an act of the will.
That means the will must act based on the way it is, and its movement will simply be a reflection of the composition of the agent. That is, unless you can choose the type of will you have, but then you'd have to create the initial state of your will, which is impossible, since you'd have to create yourself.
2.) The will determines the acts of will. How so? By determining its "desires" (i.e. by choosing which desires are to be pursued and which desires are to be rejected).
Why does it choose to go with desire-set A over desire-set B? Unless there's a fixed second-order desire to go with A over B, it's random which way the agent exercises its agent-causal power. So even if the choice is determined by the agent, it's still a matter of luck which choice the agent generates.
3.) The will is self-explanatory: Why does the will will A over B? Because it wills A over B.
That just means you don't have an explanation. If I ask "why did the rockslide happen?" it's not an answer to say "Because rocks slid down the hill." That's just changing the interrogative into a declarative sentence.