Somebody who is awake at night or traveling alone on a road, and whose mind is occupied with something other than Torah, some unimportant thing besides Torah, is liable to be put to death. He's putting his life in danger. In other words, if harm came to this person, only he's to blame because he's choosing not to learn Torah. He's giving up the protection that the Torah would provide him.
We understand this to mean that somebody who is alone at night, somebody who is traveling alone on a road, is not distracted by anything. He has the ability to focus on what's important in life, which is Torah, and he's wasting the opportunity to think about nonsense. So that person is doing something egregious. He's doing something really terrible. He has to make sure that he uses his time, especially when he has the opportunity, and not waste it. Use it for Torah.