Have to agree with you.The twist isn't what ruins him, I was expecting that to happen. What ruins him is how he conducts himself during the ending.
If he wants to rule the humans fine, all he has to do is say nothing and wait for them to come to him, with demons running around it's harder to think of someone better to put in charge than the guy who destroyed Mundus and his system. Instead he broaches the topic in the most idiotic manner possible at the worst moment possible. It's because of him that Dante develops the way he does, it destroys his credibility as an intelligent person to think his brother would have no issues with his plan. Not only that there's no reason for them to fight, Dante isn't racing Vergil to Mundus' secret mind control ray or anything that would add a sense of urgency to make fighting right then necessary, it's something that two characters portrayed as being so close should be able to talk out. It makes their friendship hollow to turn instantly to murder like that.
The ending was no better in the context of the game than it was when I watched it on youtube. Vergil's heel turn could have easily been saved for a 2nd game instead of being almost shoehorned in for the DMC3 throwback. The overall story wasn't great but I did enjoy the relationship between the twins and the banter between the two in Mission 18 is one of my favorite parts of the game. Just a missed opportunity to do something unique with this universe.