>>51181943Rose should have not been a villain, the plot should have made you help him with Eternatus willingly only for the experiment to go wrong and let you keep Eternatus till everything gets fixed.
An excuse for fighting Rose could be him being a past Champion in his youth and him being curious about your power.
But I'm sure everyone would have complained about the lack of a baddy bad. I like to think GF actually had in mind to have a peaceful plot but then changed it last minute, resulting in a plot that doesn't make sense at all.