So I think it's Odium.
Yes, Odium is heavily invested in Roshar, but he's been around the Cosmere before and I don't see him needing much investiture in order to create one spike.
Bleeder is *hateful*. Her whole motivation is driven by hatred. Once free from Harmony's control, you'd think she'd approach Wax and explain the situation - instead she sets up Wax to kill her again.