That's some pretty violent phraseology to use for a third grade teacher much less directed at an 8 year old. ( She sounds like a drill sergeant or Sonderkommando, and not a teacher.)
No doubt she has said as much to other parents about their kids as well and may now realize she has stepped over the line. (Which she has. She might as well have slapped your DS. )
I'd consider going over her head to the principal with your emails, but keep in neutral.
If you do get a F2F with the principal and the teacher is present and starts to shade or mistate the truth, then I'd have another letter with the violent phrases she has used and give it to the principal, then read it aloud. Then conclude with the statement, "That I have lost confidence in Ms XX ability to treat my child or others with respect."
