Nonsense. Lots of people were immune from blame from that war (e.g. Pope Benedict XV), for one thing, and for another thing, Austria forfeited its right to invade when it presented its ultimatum.
Capitulate to an ultimatum and your enemy no longer has the right to invade you.
What you are doing, in perhaps a Freudian slip, is admitting that Austria's ultimatum to Serbia was merely an excuse for war; that whatever Serbia did or said that Austria was still going to invade (which it did).
But invading under false pretenses is by definition not justified...and that's what Austria did.
But the ultimatum was not accepted. Read the Keegan quote. Attaching conditions and rejecting clauses is not what one does with ultimata.