For a child, that is really bad. I know there will be those who disagree with our decision to allow him play the game in the first place but when you are the only person driving with a child that cries non stop to be let out of the car seat, you do anything. Anyway, excuses or reasons or not, we need to rectify it immediately.
We tried to make him forget the game by getting him plastic toy fruits that can be separated and rejoined again by velcro, so he can 'cut' them again and again. This way, he is still the 'fruit ninja' while learning about fruits, colours and so on. It helps and he did forget about the game after that. However he became obsessed with the toys instead and will play with them all the time which is not exactly healthy either.
To reduce the amount of time he spends on the fruit toys, I decided to make up a story about a 'fruit ninja'. He likes it (actually anything with fruits will interest him) and I managed to get him to play less with the fruit toys to do and learn other things instead, like seeing and holding the actual fruit itself.
I started with easy words, Singlish in fact, just so that he understands the story itself. I would tell him each time he acts up and even use it as a bedtime story. As he understands English better, I started to use proper sentences and proper words (I try to, even though I still have a lot to correct myself) which made him question what those words mean and so on. Relentless questions are tiring to answer of course but the fastest way for kids to learn. I did that for a few months.
Recently, I started making him tell the story himself, partly because I wanted to find ways of escaping from having to tell the same story again and again (my bad). At the same time, it is a good opportunity to let him learn how to speak properly so that people can understand him better.
This is the result so far.