It depends, when the person is killed for a crime, then it is classed as execution, but what happens if it turns out later that he did not actually commit said crime? That makes it the pre-planned killing of an innocent man.
Whether execution is murder or not is very much a personal thing I feel, we can get into 'letter of the law', but at the same time, it's like opinions of the Japanese killing the Whales, or Seal Culling, or Cane Toad culling, it really depends on your point of view. The Japanese find Whalemeat useful, the Seal Cull was believed by a large number of people to be unpleasant but very neccesary, same with Cane Toads, only more so, those things are a true menace.
All of them, like execution, are considered a neccesary removal of life, but where does neccesity end and murder start?
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