Robot, the family's artificial companion
Suspected because its recreation center was found heavily damaged. A robot is not allowed to destroy its possessions unless a bug makes it disobey one of the three laws of robotic.
Its story: There was a lot of noise because of the party, so it stayed in its recreation center to watch documentaries. As soon as it heard the two first gunshots, it tried to go out of the recreation center to protect the humans, as it had to. But for some reason it would not open. So it had to break the door. By the time it managed to go out, it had heard four additional gunshots and there was no trace of the killer.
I don't trust robots!