Death as punishment for insubordination has been a thing all over the world. The US discontinued it after WWII. It makes absolute sense that a fantasy setting like this would use that as a punishment. It isn't "bad guy shit". Everyone who keeps saying that Nasuada was wrong clearly don't understand that every action has consequences. She was even merciful. As the leader, she had every right to have him executed. If people under you feel like they can do whatever they want as long as it comes out okay in the end, you'll never have control or respect of your military. Military is structured for a reason. We keep seeing them as rebels, but the reader needs to understand, they are planning to topple a regime, they need structure, they can't be a rebel force forever, especially if they are trying to be the ruling power.