Daemon is the latinization of the original Greek "daimon", which basically means full of wisdom and was a guardian spirit of sorts. I agree that formalized religion and its "demon" are missing the mark in their definition. But, I suppose it's become a good catch-all phrase for things we find unfavorable and that which goes against Judeo-Christian concepts of God and righteousness.
And personally, I think the Buddha would probably be fine with your example of a derogatory description of him. We are supposed to kill him if we meet him on the road, after all.
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