It's not a problem of grasping it, a lot of us live in societies and laws that protect us as civilians from exactly these situations so it is pretty relevant to apply the correct standards to it, not the standards US has wrapped around their people. There are regions in my county which are completely lawless, where a 5 year old kid would be equipped with a gun and allowed to use it freely and what not. That doesn't mean I should say oh ok, the police are shit and corrupt, the people in that whole state are used to others having guns in the open and as a result unsurprisingly people die from gun violence all the time.
Rather think about what your values around protection of human life are and where this dude stands as someone who had zero regards for his own or anyone else's safety doing what he did. You can defend the person in that situation saying they didn't break any laws so it's cool, or you can look at it and say people lost their lives because of one person's actions, and they should be punished for it.