No one could have known
All of the recent reviews who have been saying things like Newgroudns staff should have realized this was a warning sign for a violent person are full of crap. Hindsight is 20-20. If Newgrounds put a limit on excessive violence or reported violent animations, just about every portal artest here would have cops showing up to their house.
Think of your own animations. How many of them could be considered "warning signs" or "a cry for help"? The simple fact is that Newgrounds is our place where we can vent, talk to other like-minded people, and be creative. People who say NG staff should review submissions for possable warning signs and report them are just wrong.