The Doctor wrote:
About Sherlock starting paying attention to the grammar because he was bored: I read it differently in so far that Sherlock quickly deduced that the man actually did commit the murder. Thus he had no interest in getting the man of the charges and concentrated on correcting his grammar instead.
Yes, my comment about Sherlock seeing how long this guy could keep his cool related to that.
This guy lost his temper quickly with Sherlock over simple grammar corrections. As the guy himself said, he lost his cool with his girlfriend over something more 'delicate'. It doesn't take Einstein to work out he'd lose his cool enough to strangle her, accidentally of course.
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Also, please note that sentences can also end in full stops. The exclamation mark can be overused.
Sherlock Holmes 28 March 13:08
Mycroft’s popularity doesn’t surprise me at all. He is, after all, incredibly beautiful, clever and well-dressed. And beautiful. Did I mention that?
--Mark Gatiss
"I know that you believe you understand what you think I said, but I’m not sure you realize that what you heard is not what I meant."
Robert McCloskey