I'll throw my lot in with the "I haven't the foggiest [bleeping] notion as to why Deathwatch is Evil."
Although sadly, in my game, it makes perfect sense. I tie Alignment to universal forces, and Evil is Negative Energy. (I could just change the spell Detect Evil to Detect Negative Energy, but I find Evil has more punch to it.) Alignment is Inherent, and not tied to morality. As I explained it to my players, A werewolf will always detect as evil, even when he is saving children from a burning orphange, and a werebear will always detect as good, even when he is burning the orphange down.
Clearly though, that's not the way the traditional D&D alignment system works, so it doesn't make a lot of sense. Apparently its evil to know when your friends are about to die. (Although sadly, it hardly seems to matter since players are always yelling out "I'm down to 3hp!" when they get slammed. But there are other uses for the spell. Apparently knowing the charismatic stranger you just met is undead, or that statue is a Golem, is eviiiiiiiiiiiiiiiiiiiil!)