I get what you are saying, but there are things that can be different between two people who are physically equal that might make an attack more likely to be deadly, or give one of the apparently equal people a distinct advantage. One of them is mindset. Another would be rage. Another would be the element of surprise.
If I am sitting on a park bench feeding the birds popcorn, and an enraged person comes up behind me and punches me as hard as he can, I probably am at a disadvantage combating him from that point on, even if we are physically equal. Also an attack like that can quickly go from being non-deadly (ie, a punch), to deadly (ie, repetitively punching and kicking me once I am down). At some point, a physical assault with fists and feet becomes a lifethreatening attack.