Why does Jim (the narrator) break a glass and mess up the tap of the rum barrel?

The coarse, hard-drinking figure of Billy Bones has made himself right at home on the Admiral Benbow. He annoys the other patrons of the inn with his salty language and endless carousing; he also scares the living daylights out of Jim Hawkins's dad. But Jim does not mind him so much. In any case, dealing with such a character is good practice for Jim, as he will encounter many more of them in his subsequent adventures on the high seas.
But even Jim gets scared now and then. Billy Bones may be a bit of an old scallywag, but he is basically a decent man in a funny sort of way. The same cannot be said of Black Dog, however, a real scurvy cutthroat if ever there was one. He arrives at the Admiral Benbow to pay a none too cordial visit to Billy Bones. He wants to get his greedy hands on a certain treasure map that Billy has in his possession, and he is not about to take no for an answer.
Once he arrives at the inn, the old sea dog sits himself down at Billy's table; he has some important business he would like to discuss. But first, Black Dog wants some rum, so he sends Jim to go fetch him some. Jim does not like the look of this character; he senses something bad is about to happen. He is right: in no time at all, Black Dog and Billy Bones are at each other's throats. Tables fall, chairs fly, blood flows, and before you know it, Black Dog is sent packing.
It is a horrible ordeal for Billy; he needs some rum to calm him down. Jim goes to fetch some, but as he does so, he breaks one glass, fouling up the tap in the bargain. Jim is frightened and unnerved by what he has seen. You get some rough customers at the Admiral Benbow, the occasional brawl, perhaps, but nothing like this. It is perfectly understandable that after all he has witnessed, Jim should be feeling on edge. This is what causes him to break the glass.