John Gill Commentary 2 Samuel 1:15

"And David called one of the young men, and said, Go near, and fall upon him. And he smote him, so that he died." — 2 Samuel 1:15 (ASV)

And David called one of the young men
His servants that attended on him:

and said, go near, [and] fall upon him ;
by smiting him with his sword:

and he smote him, that he died ;
his orders were instantly obeyed. Kings and generals of armies had great power in those times and countries to execute a man immediately, without any other judge or jury: what may serve, or David might think would serve, to justify him in doing this, is what follows.