Innocent until proven guilty is a natural right that I see as a direct corollary of liberty (an event involving no physical coercion) and life (person).
Estoppel, as far as I understand, comes into play when the person is prosecuted as a criminal. Thus, as Harry said, estoppel happens as a “result of the aggression he primarily committed”. That statement is pointing out he/she has “committed” the crime, prosecution has already happened.
There are debates against estoppel and writings that support it. Here’s a link to a thread that gets into criminal repercussion inquiries further.