We don't know for sure that Morris is an antagonist. He claims to be a victim of the paw himself, and seems almost compelled to pass the paw on against his will. He seems to be trying to resist this urge when he throws it into the fire, but that might just be a trick to make Mr. White want it even more.
Morris is a shady character for sure, but whether or not you see him as an antagonist depends on whether you believe the paw has magical powers. If it has no powers, then Morris has nothing to do with Herbert's death. Even if you do see the paw as magical, you might not see Morris as an antagonist. He might have sold it to White against his will.
If the paw really is cursed, and a supposed holy man created it to make those who wish on it suffer, then we've found our antagonist. Not only is this a cruel thing to do, with no real purpose that we can see, but it also involves taking a paw from a poor defenseless monkey.