1. 恶魔般的人;极端凶残的人
If you describe someone as a fiend, you mean that they are extremely wicked or cruel.

2. 对…着魔的人;对…痴狂的人;…迷
Fiend can be used after a noun to refer to a person who is very interested in the thing mentioned, and enjoys having a lot of it or doing it often.