[shibby]

Writing over someone else's work is a form of disrespect and the cause of much conflict within the Graffiti community.

Banksy is a polarising person within the Graffiti community. Most notably because he defaced a piece by Robbo.

So yes, it was 'supposed to happen' and depending upon your stance it could either be seen as justified or something to 'feel sad about'.

[KC8ZKF]

Perhaps you could see it as justified and something to feel sad about.