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
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.