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Well, Satan=Bad, no doubt about it. That's not the same as being a co-creator, though.
Satan does his job(s). Chr*stians think he's a "fallen" angel who "rebelled" against G-d and mucked everything up. They even call him "the 'gxd' of this world." He is not a fallen angel, he is not in rebellion, and he never was "gxd" of anything. G-d is in control of everything, including evil.
The dualistic chr*stian view of a "good gxd" and "evil gxd" at war with one another is a violation of actual Monotheism.