But they gagged his daughter and bound her to the foot of her narrow bed; (line 44)
This is the first real violence we see in the poem (although we've heard about the highwayman's pistols, so we can imagine that's part of his job). There's no blood in this line, but it's still an ugly, scary moment. We know the soldiers don't have any right to treat Bess like this, but they do it anyway. These guys, the "cops," are way more violent that the robber, and we already know whose side we're on.
"Now keep good watch!" and they kissed her. (line 51)
Not all violence is physical. In this particularly icky moment, we see something that looks a little more like what we would call sexual harassment or even abuse. It's not just that these soldiers are violent, it's that they're sleazy too. The fact that they mistreat women makes them seem even lower and dirtier. This is a really good way to make us feel for Bess and the highwayman. Most people don't immediately side with the criminal, so we need a reason not just to like him, but also to hate the law.
Shattered her breast in the moonlight and warned him--with her death. (line 78)
This is probably the dramatic high-point in the poem. It's also a really bloody image. That word "shattered" is particularly harsh and raw. The speaker could have said that the bullet "pierced her breast," but "shattered" is so much stronger. It calls up images of heavy weight, broken bones and terrible wounds. It's the difference between a scene in a movie where someone gets shot and just falls over, vs. the same scene where you see the blood oozing out of the wound. Sorry, that was kind of gross, huh?