They mean the same thing. "Help" can be used with or without "to." I would prefer to omit the "to," as it adds nothing to the sentence, but it's ultimately stylistic.
It's stylistic only because the object of the infinitive is simple. Add an additional prepositional phrase such as "with axes" and it becomes unclear if the prepositional phrase is modifying "kill" or "the cannibals" if the bare infinitive form is used, while if the to-infinitive form, the prepositional phrase would clearly be modifying "the cannibals".
