I associate the word "tranquility" with "the death of my enemies when I kill them horribly and brutally." But, I don't think this is a sign of any language associations.

How something is said affects how it should be perceived. The word cruel, in you example, sounds harsh, because when it is said, it has a harsh meaning, while tender sounds nice, because people say it in a positive way. The brain works in mysterious ways, indeed.