I agree that he is a scumbag.
I agree that his claim of self defense looks extremely bogus.
I believe it is highly likely that he will be convicted of murder.
But that conviction, if it occurs, is in the future.
At the moment, he is presumed innocent until proven guilty. Denying him a diploma, by a government entity, sets a horrible precedent. Hard cases make bad law ...
This is yet another reason why I support the complete abolition of government schools. A private entity would be much freer to act appropriately; it could for example withhold the diploma pending the outcome of the trial. It's much easier for a private entity to include behavioral rules in its contract with customers and employees.
He was expelled from school so no he should NOT be allowed to graduate.