See Angad was arrested red handed, holding the murder weapon with the body. Everybody thought girl who was killed was Sahiba. But it was proved Sahiba is alive. But then who was the girl that was killed. Even if Angad didn't have any motive to kill that girl, he was still caught at the crime scene, so unless they prove that girl was killed by rumi, Angad will not be acquitted. Even if we know the truth, the Law needs evidence, solid proof. As of now there's no proof to show Rumi killed that girl. even the murder weapon has Angad's fingerprint. That is why they need proof against rumi or Rumi has to confess he killed the girl.