All men kill the thing they hate, too, unless, of course, it kills them first.

The line suggests a grim reciprocity between hatred and destruction: intense animosity tends to drive people toward eliminating what they despise, whether literally or by sabotage, exclusion, or emotional withdrawal. The qualifying clause—“unless, of course, it kills them first”—adds Thurber’s characteristic dark wit, implying that hatred is also self-endangering: the object of hate may retaliate, or the hatred itself may consume the hater (psychologically, socially, or morally) before any “victory” is achieved. Read this way, the quote functions less as a prescription than as a sardonic observation about how hostility escalates into mutually destructive outcomes.