The idea of a rival power to G-d is inimical to monotheism. It’s dualism. There is a Devil, but he’s an employee of the heavenly court, nothing more nor less.
Wasn’t even implying he was a rival power. He can only do what he’s allowed. Thankfully we have our free will to either reject or embrace the master of lies.
Employee? He was fired and pitched out on his head (along with his followers) so he’s one angry cat cause he already knows his fate. His focus is to take as many with him as possible to that fiery lake.