She isn't yelling because she doubts it will be anything Merrill hasn't already heard before. Ellana doesn't approve of blood magic at all. She thinks even if a person has good intentions that those can become easily twisted. But she has no knowledge of Merrill killing anyone to fuel her magic. And Ellana is the representative to Dalish mages on the Mage Council. She would be a pretty poor one if she refuses to represent Merrill, if she wants it.

"Really? I feel like I know Skyhold pretty well by now. Meet me at the bottom of the stairs and we'll go out together?"