The best index to a person’s character is (a) how he treats people who can’t do him any good, and (b) how he treats people who can’t fight back.

Abigail Van Buren (1918-2013) American columnist [a.k.a. Dear Abby, pen name for Pauline Phillips]

“Dear Abby” column (16 May 1974)

The earliest variation on this thought appears to be from Paul Eldridge.