They don't give a shit about your elbow nor should they. Everyone who gets put in cuffs complains about some current or former injury.
I'm not sure I buy every second of your recount. but there is a big difference between a good samaritan and a do-gooder. If the cops tell you it is time to leave the scene, it's 1 AM on a city street, you've been drinking, then it's time to leave.
Yes cops are generally assholes. The nice guys don't last out there.