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"tu" - from my french class - years ago, "tu" is used when you're familiar with someone, like you friends or family. "Vou" for someone you don't know or a teacher or superior. I don't think "tu" is automatically 'disrespectful'.

In that context, it is indeed automatically disrespectful. "Tu" is used when you're addressing a social inferior, such as a child (even a stranger) or a servant, or to a friend. You say "vous" to anyone who out-ranks you. That's why using "vous" is used when talking to strangers, the way we use "sir."

But I think "tu" is considered correct usage when talking to criminals.