fiend

The serial killer was described by the media as a heartless fiend.

(Chinese) 这名连环杀手被媒体描述为一个冷酷无情的恶魔。

My uncle is a total crossword fiend; he solves three puzzles every morning.

(Chinese) 我叔叔是个彻头彻尾的填字游戏迷；他每天早上都要解开三个谜题。