I'm sure most people know how labs are, they are great with children, but also very protective. Also the dog would be very protective of Michelle, since it was her dog?
They also bark at the littlest thing, so if someone was trying to break in, the dog would have gone crazy. I wonder if he was penned up or let out during the murder. I don't think they had a fenced yard, right? Do they have a garage?
We had a lab a few years ago when my son was little. When my husband was at work the dog was so protective of me and my baby. He originally belonged to my husband, but completely adopted me when I came into the picture, which sometimes irritated me a little, he was always in the way!!
As our little boy got older 6 mos to a year, our dog would let him crawl all over him and he would always have handfuls of hair in his little hands. He (the dog) never once growled or made any menacing move towards the baby. When he'd had enough he would go out of the room and take a break.
I don't see that dog letting a stranger get near Michelle or Cassidy. This is just based on my own personal experience with labs. Also, ours was part rotweiller, but looked and acted just like a lab. Could be why he was more protective?