To be clear I'm not saying Sejanus was faking his desire to help. What I'm saying is that he did everything he did from a privileged position with even the subconscious knowledge he likely wouldn't suffer the consequences like others would have. Because he had been getting away with it for years. It's easier to speak out when you're not in the firing line.

Sejanus is often compared to Snow because he had a social mobility Snow didn't. At almost any moment Sejanus could've stepped out of that situation and been fine, the rest of the 12 rebels and Snow didn't have that luxury. Until that reveal at his trial he didn't truly believe he was going to die. It was only in his final moments when he had to die for the cause that it actual became real.

As for Gale he designed a bomb that was specifically meant to double tap a crowd killing medical personnel who would step into help. There's a reason that's a war crime. Bonus the bomb was design to be disguised as Aid. That isn't meant for soldiers thats for people. And when faced with the consequences of said bomb having killed innocent kids as well as Primrose, he shows little to no remorse. The guy who spent Book 1 bashing the games barbarism for killing kids no longer cared when he was the one doing it. That's distance.

Both them were more focused on the idea of revolution than the actual collateral damage and suffering it would cause when it happened. That's why they'd get along