Surfin' on the Bieber hate, and that's it
It's a lot like the old Assassin games. They're not fun in terms of gameplay, and bank purely on the hatred the player has for the subject being killed/tormented/whatever.
Repetitive gameplay that devolves into click-spamming. Your reward for success is more difficult gameplay with weapons that deal less damage and have samey animations (except for the last one which has low fire rate and slow projectile speed, so you actually need to aim that one).
A positive thing is that it is short so it didn't take me long to see the end. I did not enjoy this game; I do not know who Justin Bieber is and from what I've seen on the internet, that is a good thing. The only people who are likely to enjoy this game are those who truly loathe and hate this guy.