As with any thread that carries a 'How could someone ...' message - the answer is always the same.
If you are asking that question, it's because you are assuming that there is a rational thought process at work here, and you want to know what that is.
That is where your thinking parts company with the thinking of people who do things like this.
It's pointless to look for a rational reason for this behaviour -there isn't one to find.
Yes, this man dreamed up a (facile) reason for his action, but the truth is, he wasn't actually thinking at all about what he was doing, or why, or whether it was right or wrong.
These concepts require an ability to think rationally, and with an element of intelligence. Sadly, neither are pre-requisites for ownership of a mobile phone, and access to social media.