If you were there, would you have done something when Tom hits Myrtle?
I know it's just a movie but let's pretend it's real life. You are someone who knows the people, surroundings and the couple.
You saw what Tom did after she repeats ''Daisy'' many time and draws blood on her nose.
Because from what I know, if my ex BF or his friends were there and it was real, Tom would be praying he doesn't end up with missing teeth or still has enough energy to run fast. This would obviously be overdone but that's how they feel.
If we were to analyze the characters, I don't think that's the first time Tom hit her but it was hidden back then.