I am really sick in the head for liking the ending

I'm sick too. The end was a good fit for this story, since the begining it was clear it wouldn't be happy.
Biryu was my favorite character.

I assure you, you're not. It's actually a befitting ending for the story for 2 reasons
1) Sura is a god of war whose very existence means destruction, domination and death. So, Choah's fate was sealed when he caught Sura's attention

2) Their fates were intertwined and no matter what they did, they are bound to be together.
I think if you can see it this way, it makes it easier to accept the outcome.

I’m sick to, then.
It’s a sad ending because I’m used to fluffy he, but I think this fits the story overall.
It’s more mature and serious than other type of endings.