Having watched hockey for almost a half century now there are two things that come to mind regarding this incident. 1) Sometimes freak things happen and 2) Sometimes players lose their cool and do violent/reckless things that end up worse than intended.
I appreciate your analysis, but I'm still not convinced it was a complete accident. From watching the video, I think it's very possible that Petgrave was trying to pull some kind of stunt to slow Johnson down, but things went sideways and his skate got up high.
Petgrave has a history of being a puke on the ice. Suspended in both the AHL and ECHL. Led the EIHL by quite a margin in PIM, including getting kicked out of games. Do those things mean he intentionally tried to kill a guy? Definitely not. However, there is a history of a certain amount of anger and carelessness.
No part of me thinks Petgrave wanted to kill Johnson. My feeling on the matter is that Petgrave did something incredibly reckless and dangerous, and it ended up being much worse than he intended.