What's the difference between a soldier, an assassin, and a mercenary?

[Raptor1]

A soldier, as I see it, does not work for money, but rather is loyal to the organization (usually his country) for ideological/personal reasons. You might say a mercenary is a hired soldier as an assassin is a hired murderer.

A soldier operates in a war zone, where to achieve his objective he is usually threatened by others, does that not make it more self-defense than assassination? I don't think you can really compare soldiers to assassins...