They have seen one of their own testify against him.

Americans still had the right not to testify against themselves in court, but so what?

No individual was allowed to testify against him or herself.

He is also required to testify against others in the case.

"Are you going to testify against me at the trial?"

"And the police have the authority to force you to testify against yourself?"

He's not going to meet anybody in person who could testify against him.

A wife can't be forced to testify against her husband.

He agreed to testify against any of the others at trial.

Nine people testified against him at the trial, including another wife he tried to set on fire.