No, her motivation is to break contract between Denji and the Pochita by trying to make Denji suffer. She doesn't exactly tell WHY she wants to do that. It's unknown whether the dog will remain, and there isn't exactly a reason to think she would even need it. That dog wasn't even strong in the first place. Given that Denji is dumb, it is doubtful she is even capable to inflict enough suffering on him, even if she summons every demon in existence. So... roundabout overly complicated plan for nonsensical reason. There has been a great buildup up to this point, but this is a major letdown. Oh, and in the end it is implied that Makina is the one who made Denji kill his father, and at that point he wasn't contracted to the dog.