This sounds broken and stupid, but I think it might be the system working well. I don't think we want stand your ground laws like in Florida.
Self defence is allowed, but only so far as to defend yourself. You can't go maim someone once your outside of danger. Also this doesn't mean the guy is guilty, just charged.
It's pretty nuanced: https://lois-laws.justice.gc.ca/eng/acts/C-46/section-34.html
Even the post points this out:
"What would be unreasonable," former police officer Dan Jones told CBC News Network, "is as you've got them subdued on the ground, already under control, continuously hitting them, or hurting them, or stabbing them."