This is how I saw it as well.
As the buzzer goes, Soucy is engaging McDavid behind the net.
1. Soucy pushes McDavid
View attachment 869917
2. Off balance, McDavid winds up for a slash
View attachment 869918
3. McDavid follows through with the slash to Soucy's leg with some force, but clearly held back from what he could have done.
View attachment 869919
4. Soucy responds with a chop to McDavid's leg.
View attachment 869921
5. Soucy continues the scrum by winding up for a crosscheck to McDavid's chest. Meanwhile, Zadorov takes exception to seeing his partner slashed after the buzzer and gives McDavid a crosscheck from behind.
It appears Soucy's original crosscheck was heading toward the white horizontal stripe on McDavid's jersey.
View attachment 869924
6. McDavid, not expecting a check to the back, crumples as Soucy throws his crosscheck. Soucy's crosscheck appears to connect around the chin/neck.
View attachment 869925
Ultimately, this feels like an unfortunate series of events in a scrum after the buzzer. I think McDavid and Soucy's actions are kind of a wash in playoff hockey. If you slash a guy after the buzzer, expect physical retribution. I expect the refs may have taken both players if this happened during play.
Zadorov as the 3rd man in is the problem. His crosscheck to McDavid's back is certainly worth a penalty, and he has been fined as a result.
For a suspension to be warranted, I would want to see an intentional crosscheck aimed at McDavid's chin/neck. McDavid falling while Soucy is throwing a crosscheck at McDavid's chest happens in less than a second, so I think you have to give Soucy a bit of the benefit of the doubt here.