I'm probably gonna go to hell for suggesting something as twisted as this, but based on a few things that don't fit the story:
A- Benoit was very family-oriented (he may have had a fight with Nancy but if all he did was THREATEN to hit her and trash the furniture, then he's more restrained than many)
B- I've heard rumours that the son was found with needle marks on his arms
I can only think of one thing that fits
1- Son idolises Benoit, wants to be wrestler
2- Son takes steroids on Friday (possibly finds them in father's room)
3- Mother finds son, calls Benoit home for 'family emergency'
4- Son flips out and hits her over the head, knocking her out
5- Son panics and ties her up; in steroid-induced craziness (think what it'd do to a 7-year-old's mind) kills her
6- Benoit gets home Saturday night, finds wife's body, goes crazy and kills son
7- Kills self next day