DM didn't say 'terrified', she said she was 'freaked out'. That's a big difference to me. I don't think she had a clue and was genuinely confused. We know she thought Kaylee might have been playing with the dog. Maddie and Kaylee had been texting Kaylee's former boyfriend, maybe she thought he showed up.
When living in my off campus housing during College, there was a basic rule. You don't just call 911 (snitch in the young ones eyes), that's an unwritten code for these life inexperienced young adults and probably still is today I'd imagine, especially if you're not actually seeing a murder or violence being committed.
DM was confused and didn't call 911 IMO 1) Because she didn't know what was truly happening if anything 2) She wouldn't want the police showing up if someone had a late night booty call or something silly and risk her friends getting upset with her. That would have been a logical thing for her to think, just as it would have been if I'd have been in the same situation.
A quadruple murder wasn't on her list of possibilities for the night that's for sure. I'm sure she is emotionally tormented over the whole situation and will be for the rest of her life.
JMO