Remember what the collector said:
To you who stray so far from home.
To me who's trapped beneath these bones.
We'll play forever, me and you, when you paint the land in nine bright hues!
Betrayed, beguiled, alone, deceived!
We'll have our revenge on... Unity. They also said:
"I wanna play and see everything go all Aaaah!"
"I'm starting to think you make those things just to destroy them."
- The collector is the one, who wants to k*ll all the witches! So, do y'all remember the code?
"Seething seas and puppet strings, he no longer dreams of Kings. As above rush darkened skies, as below..."
So that means, Belos is the one, who's the puppet on the strings and the Collector is playing with him. So the Collector wanted to be the king of the boiling isles, but now they want the boiling isles for themselves alone! That's why they want to k*ll all these witches with the drowning spell. But Belos is a witch hunter and he also wants to k*ll the witches.