In Satanism a well-developed ego is a positive thing. It's important to draw a distinction between bad forms of ego that disempower you and good forms of ego that empower you and help you achieve things, develop yourself and reach Godhood.
A bad type of ego is when it's seeks to gratify itself and become something using external things. It continuously seeks to get more things in order to be "someone". It's especially bad when it seeks to affirm itself based on other people's opinions and tries to gain others' approval or attention. If you base your sense of self on other, external things or people then you have zero power over yourself.
Another type of disempowering ego is an insecure ego that either tries to affirm itself by claiming it's superior or whining about "poor me" and how bad it has it and secretly tries to be superior. Both are not opposite, but parts of the same insecure ego and one form can easily change into the other. Arrogant people are usually the most insecure.
But if you have a sense of who you are and who you want to be, your perception of yourself isn't based on external things/people, you have a sense of direction and you know what you want and work to achieve it, you have a healthy, empowering ego and you should further build it, empower and refine it.
The enemy Right-Hand Path religions work to disempower you and prevent humanity from accomplishing great things and evolve themselves, because they lump all forms of ego together as unhealthy and say you have to get rid of your ego, and not just remove its weak points and build on its strengths like we seek to do. If you have no ego you are just a slave, a nothing.