MINNEAPOLIS -- There were two Black men at the scene of the police killing in Minneapolis last month that roiled the nation. One, George Floyd, was sprawled on the asphalt, with a white officer's knee on his neck. The other Black man, Alex Kueng, was a rookie police officer who held his back as Floyd struggled to breathe.Floyd, whose name has been painted on murals and scrawled on protest signs, has been laid to rest. Kueng, who faces charges of aiding and abetting in Floyd's death, is out on bail, hounded at the supermarket by strangers and denounced by some family members.Long before Kueng was arrested, he had wrestled with the issue of police abuse of Black people, joining the force in part to help protect people close to him from police aggression. He argued that diversity could force change in a Police Department long accused of racism.He had seen one sibling arrested and treated poorly, in his view, by sheriff's deputies. He had found himself defending his decision to...