Am I not allowed to construct an alien mind that evaluates morality differently? What will stop me from doing so?

No. Morality (and their rules that promote intra-group strength) is a almost mathematical consequence of evolutionary game theory applied to the real world of social animals in the context of darwinian evolution.

The outcome of the application of EGT to the nature is what is called "Multilevel selection theory" (Sloan Wilson, E O Wilson). This theory hat has a "one foot" description: within groups, selfish individals prevail over selfless ones, between groups, the group with selfless indifiduals prevais over the ones with selfish individuals.

This is the essence of our evolved moral judgements, moral rules, and of all our internal and external moral conflicts.

http://ilevolucionista.blogspot.com/2008/05/entrevista-david-sloan-wilson.html