When a society develops and gets stable enough, when there are enough officials and the order of law is strictly enforced. Revenge is institutionalised, which means courts of law are dealing out punishments to criminals, not those who were wronged personally. We can't have modern, productive societies if every single person needed to keep themselves trained and equipped to fight for their lives every single day. So, we have the police forces do it instead, while the majority of the population can study and do other work and business. It requires the vast majority of the population subscribes to this order that people can stick to high morals and consider ethics to function in everyday lives.
When the order is removed, like when a neighbouring country attacks, people have no choice but to kill before they are killed.
Btw, I already said it, but I'm not trying to sat Rudeus should become ruthless and kill every wrongdoer. I'm merely saying he needs to be ready to take a life, when there's no other realistic choice. Unfortunately the situations are usually such that you don't have more than a fraction of a second to make the decision. Similarly, he can't become shocked stiff when he sees people lose their lives. Because he's going to keep seeing it, no doubt.