I think aeternitas has been around since the brother was a kid, waiting to take over him (which I think he could only do once he was killed by Claude). My guess is that he felt inferior to Claude or something since it was mentioned he had no magical power compared to him. Which might explain his changing attitude (plus aeternitas influence)
I think that mainly it was because of aeternitas's black magic that what what made it worse his mother pressureing him about becoming the emperor which was obsessive if you read early chapters.. But I think mainly he was a good person who truly loved claude which makes everything even sadder ╥﹏╥
What happened that his attitude towards Claude changed? Did he get possessed as a kid? That seems to be the only answer.