I think the reason was simple, he enjoyed shooting the gun suppressed just for the heck of it. I personally wouldn't do it, but I am certainly not faulting him, it is fun. And even though seven other people were shooting indoors at the same time, you could hear that distinctive report every time he fired.
You know the rest. In the books you have read
How the British Regulars fired and fled,
How the farmers gave them ball for ball,
From behind each fence and farmyard wall,
Chasing the redcoats down the lane,
Then crossing the fields to emerge again
Under the trees at the turn of the road,
And only pausing to fire and load.