Ditto this. DH and I have driven through the canyons from Pacific Coast Highway (Highway 1) back to the 101 freeway many times. Calabasas, Westlake Village, Thousand Oaks are close to it, and these are fairly well-heeled areas. Although it seems remote, it still is close enough to Los Angeles, and gang members could easily drive to it. I am wondering if this could be a gang-initiation type shooting.
Also, now that it's summer, it's a straight shot to the Santa Monica Pier and Malibu beaches via the 10 freeway from some of the more crime-ridden areas of Los Angeles. I'm enclosing a link to a Los Angeles gang map, and you can see how easy it is to get to the beaches via the 10 freeway.
Los Angeles Gang Map
ETA: We've had occasional unsolved shootings along the beaches in the South Bay. I remember one guy who was biking north along the Strand (Manhattan Beach, El Segundo to Santa Monica) after midnight. He was found shot to death. I don't think anyone was ever arrested, and IIRC, LE attributed it to a gang-related shooting.
Most importantly, my heart goes out to Tristan's Family and Friends. Praying this is solved soon.