He is the Praetor of Indol. He is extremely old, despite appearances, and is the Driver of Fan la Norne and Cole. He is also the Driver of Malos, making him the Driver of an Aegis.
Amalthus went after the Architect to get answers like "is this the world you intended?". He was the only one who got there and what he found was weapons able to decimate populations as well as the key to control all Blades/Titans. And even when given Mythra, Adam was unable to become a real Aegis Driver like him.
From that, he assumed he was the messiah, tasked by the Architect to clean the world in order to make it as he intended. The Aegis and all Blades were gifts/tools given to him for this purpose. He indeed thought of himself above everyone, but still beneath god. Then when "god's bless" went to someone else, his 500 years of belief started to crumble and he meltdown.