If Aleister had intended to eliminate all magic, he must have planned to fight the Magic Gods on his own. Had he had any realistic chance of that without an exception like Kamisato Kakeru’s World Rejecter? Kamijou had his doubts, but this seemed to be the trick.

If he could not do it as a human, he would draw out the destructive power from the Magic Gods themselves.

[...]

Even if the limits of what a human could imagine had been extended 10 times…this destructive power could only be described as unbelievably great. It gave credibility to Aleister’s arrogant intent to declare war on the Magic Gods while remaining human.

Toaru Majutsu no Index New Testament vol.18 ch.4 pt.2,4