Well, the whole problem is that you need more information before you can claim that the activity they happened to be addicted to on their phone is what caused the violence rather than the addiction itself, or other possible factors like mental illness and unexpected variables.

Here's an article of a 16yo killing his parents because his iPod was taken away: https://time.com/138601/teen-kills-parents-because-they-took.... You'd be jumping the gun if you looked at his most played tracks and concluded that Bruce Springsteen or the iPod's uni-wheel interface caused violence, though I'm slightly more open-minded to those ideas.

You're getting a lot of replies to this comment saying these examples show correlation and not causation, which is a fair thing to say, but also dismissive.

A similar question worth considering is whether or not violent people are drawn to violent videogames. In my somewhat-informed opinion, they are, and that's significant.

These could also be described as examples of violent behavior by drinkers of water, for that matter.

There is nothing pointing to a causative relationship here.