I see "good reason" with the law's point of view, not morality (the victims deserve to die or not) nor literature (the motive makes sense or not; is it a good writing or not...). Which means, nothing should be considered as a reasonable motive to commit illegal acts, especially to murder somebody. While I do agree that in most of the cases, the victims are portrayed with not much better (if not worse) personality and sense of morality than the culprits, and they have done unforgivable things that make us think they don't deserve to live anymore. But morality is not everything in life. For whatever motives (be it to revenge or for money... etc.), murdering is wrong (both legally and morally) and nobody has the right to take away the life from another person, no matter how heartless and impudent that person can be. And at the same time, we should also consider the fact that the degree of which varies from case to case. Some victims' personality can be bad to the point that we want them do die immediately and think that it's "alright" for the culprits to kill them because there's no other way; while many murders, although still remain the same pattern (victims are worse than culprits), are prompted by various conflicts and disagreements that we can face everyday in real life, but killing is not the only option to solve the problem, and generosity should be considered as well. I personally think that the former is much rarer than the latter, but again, it's only my opinion.
That's why I say there's no such thing as a "good motive", even for vengeance. However, with that said, I personally do have a sympathetic feeling for some culprits (mostly from the "former kind"), because if they didn't get rid of those victims, they (or their beloved ones) might suffer from further mental/physical torments, and it looks like murder is the only way out. But since murder is illegal for whatever reason, they'll still have to take the punishment for their crime. I think that deep inside, we all know that this is the unfairness and cruelty of life, but there's nothing we can do about it, because without law, the society will turn into chaos. That's the sad part. However, I still keep my opinion that there's nothing as a "good motive", if we see it with the law's perspective. The 5 motives that I listed in my other post are worst (IMO) because even if you see them from other perspectives other than law (morality, literature...), those are still very far from being reasonable.
And of course I’m only talking about murder here, killing by accident or for genuine self-dense are different types of homicide, which rarely happen in DC.
There exist motives other than revenge actually, though not as many, but they still exist. Most robbers in Conan kill people for money, you may also find properties-related motives in some cases that involve inheritance/business/treasure hunting as well (a fine example would be the Blue castle case, where the culprit killed a series of victims just for the family’s treasure, even though many of them are actually silenced, but the final goal is still money). I'm not sure about power but jealousy is the main motive for the roller coaster case (V1), she killed the victim because he dumped her to be with another woman. In the locked bathroom case (V20), the culprit killed her sister for stealing the one she loves. While there are still some levels of revenge, jealousy is the root motive that prompts these vengeance. Those cases are only examples by the way, there may be much more.
Murder cases in Conan also have many other kinds of motives as well, such as to silence a witness (this is quite common), or because the victim blackmailed them. The BO usually kill people who they think may become a threat to them. Some people don't even have any specific reason to kill someone, they just follow the orders or are blackmailed to do so.