Because he hates Barry.I don't get why Thawne stabbed Barry's mom. He's vibrated his hand through people before, why use the knife on Barry's mom? I don't think Thawne intended to frame Barry's dad, at least it was never mentioned, and thawne said he killed her in a fit of rage since he couldn't kill Barry. So why use the knife?
Anyways I hope we see killer frost in season 2