Angels and devils undermine monotheism which is God alone.
A slight modification-— they exist because He created them within the system of creation. A slight tangent:
When you hear “Kabbalah” don’t think about Madonna, please. It is the teaching of how an all-powerful G-d can “contract” Himself down into a tiny physical world (our universe.) There are infinite worlds in between His essential state and our planet. On one plane are angels.
So they exist, but we are forbidden to worship them..that undermines monotheism.