Treacherous has two related but distinct uses. About a person, it means willing to betray those who trust them — disloyal in a dangerous, harmful way, not just unreliable ('a treacherous friend' gave your secrets to your enemies). About conditions, especially weather, roads, or terrain, it means deceptively dangerous — the danger isn't obvious just by looking, which is exactly what makes it dangerous ('treacherous ice' looks harmless but can make you fall badly; 'a treacherous mountain path' looks walkable but hides real risk).