My MSc advisor gave me an idea that was already done and was not expecting me to add anything new but I found new insights, he was not convinced, he never helped or gave me motivation but, I continued anyway and wrote a paper, presented it at a conference, and gained positive feedback and I will publish it but now he changed his mind and want to put his name on the paper!
Is it even ethical to do that?
It might not be ethical, but it might be common practice and the expected behahiour in your team/lab/department/college/university. There are many unknowns here and it's difficult to give a specific, applicable response. Nevertheless, here are some things that you might want to consider:
If you go ahead and publish alone over your adviser's instruction, there is the possibility that the adviser will write to the journal and contest the authorship of your work. Then it gets really messy. For example, visit COPE's database, which is full of case studies. I searched on the key word "supervisor" and came up with some interested parallels: https://publicationethics.org/guidance/Case?t=supervisor&sort=score
There might be other, less direct costs to you and your future career if you proceed against your adviser's instructions. You need to weigh this yourself. For example, any future association with your adviser will have this hanging over it. This may or may not be relevant to you. For example, you might be moving on to a corporate job and not heading to academia.
Whatever you choose to do, you need to be able to live with the consequences of your actions and your sense of integrity and fairness. If your personal ethical stance is one of moral absolutism, then your path is clear. If it isn't then what parts of the context or consequences are bothersome to you?
I would be careful about broad statements like "he never helped or gave me motivation". Your supervisor will probably have a different view on this. I'm not saying that it's not true. I'm saying that it's very hard to provide evidence of absence.
Good luck.
What is ethical and what is possible might differ here. Given your description it was probably improper for him to ask. Perhaps not, depending on how "mature" the idea was when you got it.
But telling you to, for example, read a paper for insight isn't a real contribution to the resulting paper you wrote.
But it is often a career killer to go against your advisor in such things. If you are beyond his control the decision is yours alone, but if he is in a position to hinder your career you might have accept it. And move on ASAP. Tread carefully until you escape.
I've assumed you aren't in a field in which advisors are always added no matter the contribution or lack.
