Don't think of it as individuals, but as individual genes. A group of 10 with the same genes, that can eliminate a group of 10 with different genes by losing one individual (because they were fighting to the death, while their opponents did not) is 9 copies up.

An alternative view is that in groups with alphas that father most offspring, and status is based on the individual's ability to risk death. Genes in an individual of low status are already 'dead' so manufacturing instincts and hormonal responses that increase violence does not have a downside.