Generally speaking, just because something is legal doesn't mean it's considered moral; while every society has its share of taboos, not every given taboo is explicitly illegal. While some understand and hold this idea, others have no qualms about performing even the most reprehensible actions under the reasoning that so long as there is no written law forbidding it, it's perfectly okay.

The mentality of people who follow this trope can vary; a law-abiding villain