I'm kinda split on this. If higher levels just kept killing the lower levels then it would be no fun for the lower levels and they would stop playing and it would just be higher levels. If higher levels couldn't kill lower levels then lower levels could fight amongst each other and gain levels to fight the higher levels.
This gave me an idea. Maybe the game should be split so levels 1-10 fight against each other. 11-20, 21-30, etc. could fight against each other so it would be fair? I'm not really sure.
It would be fun either way.