Unless you’ve dressed/butchered large animals you might not have a feel for how much strength it takes to thrust/slice/stab with a large knife especially if it has a heavier profile blade like the reported “Kabar” style would have had. And one doesn’t just point and stab. You also have to restrain/hold or otherwise pull back on the target to get penetration. It’s a two-handed job.
The father of one girl said she was “sliced” in the back-—thru a lung and liver. That requires two hands. If nothing else, watch a butcher work sometime——it’s a two handed job and the cow/pig/goat/sheep isn’t fighting back. In this case it seems while one was being stabbed the other was resisting, based on reports of “defensive wounds?”
So, yeah, there’s a bit of a skill set involved. And, oh, yeah. Getting the knife sharp is another minor skill. Having sold thousands of knives over the years I can say that very few are completely sharp off the rack. Manufacturers do that to keep casual customers from needing bandaids before they get home. Anyone planning on doing damage with a knife would try to fully sharpen it before going to work.
Without a bit of experience cutting up meat, etc. it would seem likely that the killer probably knicked himself. To demonstrate for yourself, try picking up a 7” knife sometime and visualize how you would stab or slice a moving target with it. For half the strokes the blade will be pointing at yourself. So, yeah, there’s some skill involved.
When arrested he asked if anybody else had been arrested. Was there an accomplice?
Those "skills" are learned by living. I'm sorry, I forget how isolated and pampered the metro dwellers have become.