[@Turboshitter] Honestly, I kind of see the Grimm as creatures of literal darkness, in that when they die, they just completely disappear. No trace, no nothing. Like the light is just entirely taking over where the darkness once was. Or something like that. I dunno, it's kind of hard to really understand the Grimm. 
What if Salem took a bite out of a Grimm, and that's why she looks the way she does?