According to the entry for Sloane sequence A001969:
Evil numbers: nonnegative integers with an even number of 1's in their binary expansion.
And deeper inside that entry it states that
The terms "evil numbers" and "odious numbers" were coined by Richard K. Guy, c. 1976 (Haque and Shallit, 2016) and appeared in the book by Berlekamp et al. (Vol. 1, 1st ed., 1982). - Amiram Eldar, Jun 08 2021
What did Richard K. Guy see in these numbers that caused him to think that "evil" is the best description for them?