What is the ruling on electronic games that involve killing to advance through stages, and are they forbidden?

If the killing in games is unjust, like the actions of criminals, then one should avoid these games due to the corrupting educational and moral effects they entail, especially for young children. However, if the killing is just, as in righteous wars and repelling an aggressor, then it does not contain this corruption, and it should be limited to an age that is appropriate for it and can comprehend its meaning and objectives.