Other thoughts ... somewhat graphic:
LE, and I suppose most of us here, suspect the MC murder to be personal in nature.
Let's play it out at a high level ...
Perp's initial rage point: blunt-force, which might not have resulted in immediate death.
Perp proceeds to strangulation now that his victim is fallen and / or moderately incapacitated. Now it's truly hands-on personal.
Perp's satisfaction rises. But, strangulation-to-kill is difficult because MC is young, [assumed] strong and able to struggle to some degree.
Perp's rage intensifies because MC is making it more difficult for him to achieve to an ultimate satisfaction. Now it's not only hands-on. It is frustrating because of the struggle.
Perp proceeds to stab to achieve an ultimate satisfaction. A final rageful thrust, a pain-inflicting intrusion into the body of his victim, to ensure death.
I expect gaps in that overview but I assume the general progression is accurate. I previously posted evidence (other crime descriptions) that indicates that progression of attack is not uncommon ... blunt-force, strangulation, stabbing.