TIL the soldiers convicted for firing into a crowd of protestors at the Boston Massacre were branded with M’s on their thumbs.

That's a horrible source, but in any case the jury didn't believe any of what you think happened, but did believe adams and that the sentinel tried to escape to the custom house but it was locked then called the main guard commanded by preston who marched over to save him then were attacked and he never ordered anyone to fire (the crowd was shouting it, along with 'they won't fire' and 'kill them'). this is a great source for a lot of historical trials done by a kentucky law professor and has adams' summation https://www.famous-trials.com/massacre/208-summation and Captain Preston's deposition with the crown witnesses against him below it which contradict him and agree with you but the jury evidently didn't believe https://www.famous-trials.com/massacre/210-evidence

The mob still increased and were more outrageous, striking their clubs or bludgeons one against another, and calling out, come on you rascals, you bloody backs, you lobster scoundrels, fire if you dare, G-d damn you, fire and be damned, we know you dare not, and much more such language was used. At this time I was between the soldiers and the mob, parleying with, and endeavouring all in my power to persuade them to retire peaceably, but to no purpose. They advanced to the points of the bayonets, struck some of them and even the muzzles of the pieces, and seemed to be endeavouring to close with the soldiers. On which some well behaved persons asked me if the guns were charged. I replied yes. They then asked me if I intended to order the men to fire. I answered no, by no means, observing to them that I was advanced before the muzzles of the men's pieces, and must fall a sacrifice if they fired; that the soldiers were upon the half cock and charged bayonets, and my giving the word fire under those circumstances would prove me to be no officer. [somewhere below is testimony that an officer would never order fire with bayonets attached] While I was thus speaking, one of the soldiers having received a severe blow with a stick, stepped a little on one side and instantly fired, on which turning to and asking him why he fired without orders, I was struck with a club on my arm, which for some time deprived me of the use of it, which blow had it been placed on my head, most probably would have destroyed me.

On this a general attack was made on the men by a great number of heavy clubs and snowballs being thrown at them, by which all our lives were in imminent danger, some persons at the same time from behind calling out, damn your bloods-why don't you fire. Instantly three or four of the soldiers fired, one after another, and directly after three more in the same confusion and hurry. The mob then ran away, except three unhappy men who instantly expired, in which number was Mr. Gray at whose rope-walk the prior quarrels took place; one more is since dead, three others are dangerously, and four slightly wounded. The whole of this melancholy affair was transacted in almost 20 minutes. On my asking the soldiers why they fired without orders, they said they heard the word fire and supposed it came from me. This might be the case as many of the mob called out fire, fire, but I assured the men that I gave no such order; that my words were, don't fire, stop your firing. In short, it was scarcely possible for the soldiers to know who said fire, or don't fire, or stop your firing.

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