He wasn't murdered, he likely died due to the fact that he looked into basically the primeval current of magic, this has had very large effects on even the greatest of sorcerers and thops was just a rookie, he barely even knew the most basic of spells but he looked into that current and died making something entirely new, this category of magic did not exist yet so he just casually created a new form of magic
I always assumed the other sorcerers killed him to prevent his discovery from getting out. Lodged into shields, it would have revolutionized magic defense to where any soldier could master it. The Carian ash only does damage if you're already high int and isn't useful against incantations. The others don't stop spells. But Thops changed the game. Now literally anyone could shut down spellcasting.
The secret is that is deflects both sorceries and incantations. Carians knew the truth about the Primordial Current (the fact that it in itself is an outer god), but the academy did not. The sorceries and incantations were thought to be 2 different things, when in reality they were one and the same - the usage of power of an outer god. What the barrier does as a spell is less important than the fact that the first sorcerers were cultists/monks/priests of the current of stars. It being a god doesn't mean it is concious or has any devious plans as what is and isn't god isn't strictly defined in elden ring and it might be that they are certain laws of universe that define what is and is not real (there's more to my theory but I'll stop at that)
I think it’s bugged but if you hold the button to spam it it will deplete fp to zero but allow you to keep casting until you release the button. Can’t be cast at zero and the continuation costs 7 still, but will continue to cast at zero