Preparation only works (If I read it right) when combat drops, not when you get a kill. If it was when we got a kill we could just use combat focus and spam slash and kill mobs and rinse and repeat lol. Dropping combat is difficult in large group based pvp,
Which is why I specified picking off a ranged DPS unsupported by his team in the back as a scenario where Preparation would come into play. In that scenario, you'd drop combat after he dies.