Parvel never treated sion horribly. He actually loved him so much. Unknown to sion some how, parvel was there when sion was about to die. He was the one that caused the explosion that happened in the early chapters when siennas wanted to steal shion's stigmata. Parvel saw sion and they hugged, cried and even kissed before parvel went back to the past again. But somehow, Sion's memories are incomplete because he doesn't remember all this that happened before he died in the previous timeline.
Spoiler
.....................it turns out that shion isn't the only person who came back from the previous timeline. Parvel also came back. He even came back earlier than shion. Parvel never betrayed shion. He loved him in the previous timeline. The reason why parvel seem so cut throat and vicious right now is because this isn't his first timeline. And parvel has always known that the current siennas is shion from the moment he saw him on the coronation day. This story is gonna break my heart (/TДT)/