There MUST be a source for evil just as there is a source for Good.
I've never heard philosophers discuss Satan as an entity.
So many of our enduring beliefs are still with many of us after centuries of wondering and even fearing this or that supernatural being.
It's not clear why "idealists" (who idealize whomever) alone have been selected in this case, who may or may not be interested if Satan is or is not a mental construct. Idealists do tend to idealize, but are also capable of thinking otherwise, like non-idealists.
Only God exist, and is beyond good and evil. Opposites are part of the illusion of duality, so good and evil create each other.
If an idealist believed in a Satan I believe the answer would have to be yes....
Sure. The most extreme idealists think reality itself is mentally constructed, and that nothing exists outside of the mind.
In some of the philosophies that are known as idealism, EVERYTHING is thought to be a mental construct in the mind.
Is that a construct anywhere else but within our minds?
If I say.... The snow is a white blanket. Time is money. Life is a roller coaster. America is a melting pot.... do you think I'm actually saying the snow is a blanket? Or time is literally money? Or life is actually a roller coaster? Or America is actually a pot?
The problem with people reading religious texts today is they don't know the difference between metaphor and truth. They literalize everything because there mind has been trained to operate that way. This is why the Bible and religious texts makes zero sense to almost anyone. Metaphor was the common language a while back, perhaps due to limited words or perhaps more likely it was just a way of the times.
Today, you have a literal mind reading metaphorical writing. Everything is misunderstood.
Satan is not a mental construct but the personification of the force that causes you to devolve rather than evolve.
If you give into Satan, you give into animal instinct, selfish behavior, and less evolved character traits.
As you can see, Satan rules this world.
There's nothing about idealism that necessarily precludes belief in Satan.
Satan is a mental construct in your mind. My mind is occupied with things that are real.
All is made from the same One image.