Former Minneapolis policeman Derek Chauvin betrayed his badge by “grinding” his knee into George Floyd’s neck during a deadly arrest last May, a prosecutor said on Monday at a murder trial that is widely seen as a test of the US justice system.

Chauvin’s lawyers responded by saying that the former officer was simply following training from his 19 years on the force, even as they acknowledged that the arrest, caught in videos from multiple angles, was distressing to watch.

“The use of force is not attractive but it is a necessary component of policing,” Eric Nelson, Chauvin’s lead lawyer, said...