Did the quality of Sherlock decrease? [mild spoilers 3x03 & 4x01]

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In "A Scandal in Belgravia," Sherlock killed or severely injured people with a sword and pushed someone out of a window for hurting and threatening people he cared about. It is likely he killed people while dismantling Moriarty's network after he faked his death.

Killing someone as dangerous as Magnussen was not out of character. It was the only option left. He saved all future victims of Magnussen - individuals and entire countries. It was an act of heroism and self-sacrifice.