I think so too. I'm so tired of the "but they're children until they are 25!) No, there are benchmarks to hit, and at 18, we expect more common sense, ethical development and critical thinking. I realize that we're probably not at the peak of our game in raising adults in North America, but...even so, these two are really at the tail end of some curve, for their age group.
I like look at Kohlberg's stages of moral development and here we seem to have the earliest stage. No idea why you should do the right thing (or might do it). The only reason to behave well is that you might be punished. If you can evade punishment, go ahead and do it (if you want to).
Even then, a dose of common sense can be helpful in preventing people from killing each other. I just don't get it. But I know people who say they grew up around people like this and that there are more of them than I think.
OTOH, if drugs or mental illness are involved (brain chemistry), then all bets off. I've seen some crazy stuff done by people who are in some form of psychosis. Nothing like this, of course, but goes beyond immaturity.
I just can't tell, at all, how this all got started, don't know how to begin to analyze the causes.