Stupidity: For a brilliant criminal mastermind, Cyrus makes so many idiotic mistakes. (1) He leaves his entire escape plan hidden in the wall for the guards to find later instead of trying to dispose or destroy them (2) He makes the fatal mistake of trusting Santiago completely and never suspecting a double-cross. At the very least, he should have checked all of Lerner airfield just in case the plane was hiding (which it was). (3) He never notices or even questions Billy Bedlam's sudden absence until he sees his body. Odd considering he notice Santiago's absence immediately. And (4) When escaping on the firetruck, he (pointlessly) chooses to ride on the ladder instead of the front seat where he stands out, and Poe and Larkin both spot him. No surprise his whole plan failed.
Stupidity: After Nick discovers the parole letter about Po, he crinkles it up and throws it down. After Po kills him, wouldn't you think the first thing he would do would be to pick up that letter and also take the letter that his daughter wrote and put them in his pocket so that no one else could discover them? (01:01:00)
Chosen answer: Zero. As you said, he was attacked and there are witnesses that he tried to avoid the fight and the killings were in self-defense. It is an extremely weak plot hammer to get Poe onto a plane full of criminals. It's foolish as well. The writers could have had Poe framed for a crime then exonerated and put in the same situation much more believably.
Grumpy Scot
It's in Alabama. People are put in prison here for much less.
First, Poe is a federal prisoner, not subject to State laws or legal procedures. Secondly, he is not in Alabama. During a conversation with Billy Bedlam we hear that he is incarcerated in the "Q" - prison slang for San Quentin in California. It makes you wonder why a Federal prisoner is in a State prison, but that's another type of mistake.
Would it really be considered self-defense, though? After he beat the guys to the ground he could have just stopped and walked away, but he didn't. He kept beating them until they died.
He is defending his wife against two armed assailants, and use of lethal force is allowable. No DA in the United States would even think about pressing charges, knowing full well a grand jury would throw them out in a second.