THIS is why they are the best sherlock/watson he has FULL faith in her ability to brain a man and that is ESSENTIAL to their dynamic
Sherlock three and a half seasons later, when they actually already tangled with putting their minds to the idea of killing Michael (and that was before he fully attacked and tried to kill Joan, giving her even more motive) and a few episodes later he's found dead in a way that we later learn was him being brained to death by a baton (basically a mental tube), showing clearly in the way he asks that he has no doubt that it's a real possibilty and is fully just checking his facts about the crime so he'll know how to best preceed in keeping her out of jail: "so, Watson, did you do it?"