Since nobody in DR immediately turned themselves in after committing a murder, the 'everyone else might die' applies to even the failed attempts. For example, suppose in chapter 4 DR1 Hiro picks up a bottle and smashes Sakura on the head and succeeds in killing her. We know he feels sorry about the incident, but he also tried to cover it up. In this case, he'd obviously get discovered, and is he somehow a worse person because he accidentally succeeded in killing Sakura versus what actually happened?

Or let's say we switch Hiro's shoes with Kyoko. Now, Kyoko probably wouldn't agree to meet Sakura alone in the first place, but suppose she does, she might end up with the same reaction if she thinks Sakura is trying to kill her. So let's say Kyoko also succeeds in killing Sakura against all odds, and since she's the Ultimate Detective she also knows what others will be looking for and successfully destroys all evidences, which eventually results in everyone else dying. Is this hypothetical Kyoko a worse person than the hypothetical Hiro case just because Kyoko has the skills to successfully cover her tracks after an unintended murder?

I don't think the murders are justifiable, but I think there's a tendency to let things like capability or plot convenience get in the way of intent. If Kyoko or Byakuya committed a murder, it's not any less justifiable than someone else doing it. Just because they're the two who probably can get away with it and result in everyone else dying doesn't mean they're worse than the guys who had no shot at getting away with it. Likewise just because someone failed in their murder attempt doesn't make them any less liable compared to someone who was successful.