Is Satan a demonic angel? Probably not. It's unlikely that Satan and his demon followers are real angels. There are many ways to interpret the Bible. For every verse taken literally, one can find a symbolic meaning. It's unrealistic to believe that a God of love would allow a rebellious angel to inflict unimaginable pain and suffering upon people for thousands of years.
Satan is the Prince of Darkness for good reason. Light is real. Darkness is not. Darkness is merely the absence of light. In other words, when people are tempted to stray from God's love, they find themselves spiritually dead, - tormented in a world that feels like hell. In that sense, Satan appears real, when in fact he is not.
A good example of this is darkness. It appears very real. What we can't see can be frightening. However darkness is not real. Neither is Satan. e.g. When you walk into a room you will find a 'light' switch but you will never find a 'dark' switch. Why is that? Because light is real. Darkness is not. Darkness is merely a place without light. In a spiritual sense it's a place without God's light.