Minors shoot and kill people all the time, too. In US culture and jurisprudence, we tend to weigh heavily factors that diminish offenders capacity to distinguish right from wrong, and teenagers have both diminished judgement and impulse control capabilities that are at their lifetime low.

We generally try, at least ostensibly, to recognize that long sentences serve no practical purpose when applied to someone who will in just a few years be for many purposes a biologically different person.

How does "tried as an adult" fit into that view?

I've never understood why you would go to the trouble of having special juvenile criminal sentencing ranges and juvenile courts when oftentimes a juvenile does something really bad he is tried as an adult (even more so in the UK, it seems). It would be more straightforward to just exclude murder and other grave crimes from juvenile court altogether.

Yeah, it's pretty fucked up.

Minors are more susceptible to change and will likely end up being different people by the time they are 30, while if you had a much older perpetrator the mentality is probably more ingrained.