
I don't read the light novel but i got the feeling like anastacius is a good guy actually but got manipulated by the spikey hair guy..idk it just my feelings (?)

He is a good guy at first, but I don't think it's entirely Aeternitas fault that he turn the way it is, Aeternitas may possessed him and whispered him to do this and that but he still has his own free will at least till the chapter when he brainwash Ejekiel father, at that point Aeternitas already took control of him completely. From the flash black, His mother also manipulated him that he should be wary of Claude etc, not to mention after they grow up, Claude is stronger than him, so well jealousy also play a part
He is so cuteeeeeee