The basic concept here is that, when Thorax get arrested and imprisoned, despite Spike's best efforts, instead of falling into a depression or anything like that, Spike gets angry. I can see this going one of two ways:

A) We all know what greed does to a dragon, but what about anger? What about hatred? Spike ends up growing into a full-sized dragon again, except this time not due to greed, but anger and a thirst for justice or, failing that, revenge.

B) Blinded by anger, Spike decides to capitalize on his celebrity status and turn the Crystal Empire upon his former friends.

Concept A is making me think of the hulk.

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https://www.youtube.com/watch?v=modsjYkxnCA

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Or, being a civilized being, he could work within the system to get Thorax released peacefully. There are other forms of jurisprudence beyond vigilantism, you know. Unless "imprisoned" is a euphemism for "executed", he has the luxury of exploring other avenues before resorting to violence.

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Come to think of it, I haven't seen that angle either. Interesting idea there, a bit of political and courtroom intrigue.

Also, to further explain scenario B, Spike is not really the sort of guy to turn an entire empire against his friends, no matter what they may have done, it's just that he was so mad at the time that he acted before he gave it any thought. He'll feel pretty bad once he cools off and he'll try to settle the situation peacefully.