In the United States, I would strongly expect that an accurate depiction of historical fact (even if uncomfortably graphic) would be protected under the First Ammendment. Otherwise, the government could functionally censor the worst parts of history (as being too awful to discuss or depict), which is exactly the kind of thing the First Amendment is designed to prevent.
There are three important categories of speech that are not protected: (1) "fighting words" directed at a person intended to provoke a fight, (2) words that infict emotional distress such that it qualifies as a tort, and (3) speech that court finds to qualify as "obscenity". Of these three, your game probably will not qualify for the first, since it generally requires speech directed at a specific person or people. I also suspect (less confidently) that an emotional-distress tort would not succeed since your game is not directed at any particular living people. Even if the game caused emotional distress to someone, your public release of the game probably could not qualify as a tort against that specific player who happens to experience emotional distress.
The Miller test is used to determine if a work is obscenity. Wikipeida summarizes its three parts, all of which must be satisfied to constitute obscenity:
- Whether "the average person, applying contemporary community standards", would find that the work, taken as a whole, appeals to the prurient interest,
- Whether the work depicts or describes, in a patently offensive way, sexual conduct or excretory functions specifically defined by applicable state law,
- Whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.
The first two are explicitly sexual in nature. I don't know if there is any similar prohibition against hyper-violence, but even if there were, as long as your game does not run afoul of the "lacks serious artistic value" condition, you will be on the safe side of the line.
Note that none of this stops anyone from initiating legal action against you (which may cause headaches for you); it only stops those legal actions from succeededing.