1. Luffy told Ace that they'd have to fight to the death for One Piece.
2. Luffy has stated, twice, that fighting Shanks is part of his MO for becoming PK.
3. His "friendly" relationship with the crew means nothing when the stakes are about finding the titular macguffin
4. Usopp's relationship with his father is actually pretty unknown. We dont actually know if he truly loves or hates his father for abandoning him. They CAN fight.
5. And most importantly, you are taking into the assumption that you KNOW Shanks' true motivations, especially when they are called into question after talking to Gorosei, the very people who are trying to kill VP and destroy Egghead.
Stop assuming "bad blood" is the main drive or that there wont even be bad blood by the time we get to it.