Well, the reason why they don't do that is cuz theyre trying not to have more eyes on them. (Unless you're talking about Noel, of course.) If they did kill, for example, the descendant daughter of two very famous magicians, they would be EVERYWHERE on the news and eventually would end up being tortured/experimented/taken advantaged of by world powers for eterntity for their immortality.
Plus, this isn't even the first time this has happened to them. At this point, they don't really have a reason to kill a bunch of people who are essentially the equivlent of ants to this immortal family. What's the point when they'res just gonna be more assholes around the corner anyway?
It's hard not to have your cliche expectations met, I know. But everything has a reason in the perspective of these guys.