I tracked down my son's online bully and called his mother

This kid was someone my teenager had never met in real life. He was a friend of a friend of a friend online. ugh. Exactly the stupid online behaviour I had hoped they would avoid.
So, the teenager in my life got a few pretty toxic online messages. I'm generally fine with banter, sarcastic humour, and other such comments. But this was different.
This was a bit more vicious and involved threats like 'I'll come to your house and stab your dad in the neck and burn your house with your brothers and sisters inside and I'll kill your mum'.