Killer and Assassin

In fact, I don't quite understand the difference between a killer and an assassin. Maybe they are the same. What I want to express is that perhaps killers or assassins are also needed in our times.

What caused this feeling is a new movie I watched today, In Bruges (Killer No Holiday), which was filmed this year. It does not belong to the "really wonderful" category. Actually it has some gray areas and the ending is not very comfortable. Instead, it makes people feel somewhat disheartened. Yes, it is the feeling of total despair. Somewhat like that.