Today’s opinion doesn’t recite the facts of the underlying offense in any detail, largely because the legal issue on appeal (precommitment custody credits as a applied to the maximum term of confinement) does not depend in the slightest on those facts. Nonetheless, here’s how the Court of Appeal briefly describes the offense:

“This case arises out of a shooting in which Jose and an
adult man fired multiple gunshots, killing the victim.”
As a minor, the sentence imposed on Jose for his offense is . . . .
Three years in juvenile lockup.
Now, I understand that Jose was only 15 at the time, so his sentence reflects that fact, and also that this was a negotiated plea. It might also be the case that Jose ends up spending less than 3 years — if there’s a finding that he’s rehabilitated, with reviews every six months — and that if he’s really in bad shape, his absolute maximum confinement is 6 years (with credit for a year served, so five extra years total at the outside).
All that said, if 15-year olds get three years in juvie for killing someone, you can readily see why gangs might elect to employ minors as their killers — as they apparently do.