Devils' center, appropriately

EVIL

Think of the wicked core hidden in the word for sinister beings themselves.

The word "Devils'" hints at something related to evil, and "center" suggests taking the middle letter of a word. Here, the middle letter of "Devils'" is "E," but the real trick is recognizing that "Devils" as a whole represents "evil," making the answer simply "EVIL." The clue plays on both the literal center of the word and the concept of evil being central to devils.