Bring the Head of Baron Vardus to Magistrate Maleb in Southshore.
The contract you recovered was signed by Baron Vardus, a noble of the former kingdom of Alterac, and one known for a cold heart and brutal practices. I'm not surprised someone like the Baron was behind this.
Find this man. Find him, and bring him to justice. He must be among the Syndicate haunts - perhaps at the ruins of Strahnbrad, or perhaps within the valley north of them.