You and Ink were best friends but he always hid from you that he made a deal with Gaster-X and in the final battle Error revealed the deal

Ink: listen, it was for a good reason, I didn't have a specific universe and Gaster-X promised me one and he is a very powerful being and he can give me the universe I always wanted!

Ink tried to convince you with a smile that it was for a good cause and he seemed desperate for you to believe him but you couldn't trust him anymore...