A killer: Someone who takes another person's life, deliberately or by accident, legally or not, no matter the motives,
A mercenary: Someone trained in military and combat skills with no affiliation to a specific country or organisation, who is paid by contract to fight in a war zone. Morals are optional.
Also, in the world today there are people who pose a credible, viable threat to countries half the world away through organisations and technology - as such, soldiers are often the ones who are sent to deal with that threat posed to their country from foreign soil. They are still acting in defense of their homeland - to leave that threat alone would mean exposing yourself and your people to further attacks.
The only exception to this is when the basic human rights of people are under threat, or if major atrocities have been committed: then they are intervening on the behalf of those people, because they have the ability to do so.
Sometimes, the cold hard truth is all that's needed: if a credible threat exists that can and will likely harm others, remove that threat by any means.