Theory:
Young 22 y.o. guy (Saretsky) has a crush on mom.
Saretsky doesn't like the closeness between Mom and Dad, and their kid is the common bond.
How do you win the affections of the woman you are smitten with ?
Eliminate the competition......stab him to death........a rage killing.
Maybe Mom even quit seeing him because of his jealousy and nonsense.
Hailey was probably sleeping in the same bed as Dad when he went in to do the deed, and she ended up being collateral damage......then again..........Saretsky could have been striking out at Mom too.
This whole thing revolves around the relationship between Mom, Dad, and Saretsky.
Chances are good that Hailey isn't with us anymore.