Mob justice is what happens when governments fail to protect their citizens. As, for example, they failed to protect Rhiannon Whyte from a fairly obvious risk that even the dimmest person in the world could have forseen and which absolutely should have been prevented.
It's what desperate people do.
Maybe we shouldn't allow lots of illegal migrants in without detaining them first and checking they're not mentally ill or evil enough to do such things or, in fact, criminals with lengthy records in other countries who've conveniently ditched their papers. Or people who've been denied asylum in other EU countries for good reasons as has been true for some of the recent child rapists. Why is our government prioritizing freedom for these men who we don't know over safety for everyone else?
Also RIP Rhiannon Whyte, as again, if that man had been detained she would still be alive.
It doesn't matter whether or not this man was illegal or not. If this stuff keeps happening it's a good reason to make sure who is coming into our country before giving them freedom to roam and attack others because we really don't need more of this.