Sometimes 'murder' is justified, like when you're protecting civilians against pirates. That's the main use-case I see here. Nobody in their right mind would send these guys against an armed vessel.
Killing and murdering can be two different things, which is why we have separate words for the actions. You're conflating the two things as though they're always the same.
To kill is not inherently to murder. Typically from a legal and moral perspective killing someone in self-defense is not murder, for example.
The US Navy is the largest peace keeping and rescue organization in the world. Just one aircraft carrier is a disaster response super hero, complete with a hospital, food, clean water, and tents.