Why? why? why?

I was so happy last couple of days seeing the maturity in ArShi relationship. Why did they have to spoil it by making Khushi believe in Shyam. Khushi is the only person who has seen the evil side of Shyam from direct experience. Even though she has a heart of gold how can she say that? That one sentence made me feel more upset than Monday's episode. I don't know why? She could overlook the fact the he tried to kill her but he tried to kill Arnav!!!! Who is more important to her than her life. How can she forget that? Also how much can Arnav take? He's being punched by all sides!!!