Why did Rem kill Subaru in Episode 6?

Rem killed Subaru because he smelled like the witch, which she had personal history with.

Who killed Subaru in his sleep?

8 Frozen By Puck A Third Time — Puck killed Subaru as punishment for killing his daughter. Subaru and Rem set out to get to the mansion as soon as possible. A Mabeast called the White Whale appears, and Rem stays back to fight it, vanishing from everyone’s memory in the process.