Assuming that people who buy casual games don't buy violent action games, so what? If Alien Shooter is so successful, then why would another game that is better than Alien Shooter not be successful, as long as you sell to the right people, i.e. the people who enjoyed Alien Shooter, not the people who enjoy Bejeweled. So yes, I think an Alien Shooter clone would sell if a) it's better or at least as good as Alien Shooter and b) you let the right people know about it and c) the pixies aren't acting up. Not everyone enjoys match-3 games, obviously.