You are working from an incorrect assumption about your choices.
* Doing nothing is unjust.
* Providing material aid and luxuries to the slave holders/cannibals is unjust.
* Using your presumably superior force to stop murder is not unjust.
* Doing nothing is unjust.
* Providing material aid and luxuries to the slave holders/cannibals is unjust.
* Using your presumably superior force to stop murder is not unjust.