Long story short, M gets the money in games like PG13 gets the money in movies.
Microsoft asked Bungie to T down Halo 1 after they were part way through implementing dismemberment to the models and all that. The ESRB found out about the flood, and their dismemberment centric combat, and they forced an M on the rating. Microsoft went “fuck it, do what you were gonna do” but it was too late and there was just time to turn the blood back on in the midst of all the other finalizing shit.
But yeah, Halo 1 was going to have legit dismemberment through and through. On Halo PC you can still find some gore’d crewmen/ marine textures. The grunt and elite materials have detailed neckholes in their texture sheets too.
I’m pretty sure they’ve kept the feature out of main releases ever since because it makes overseas sales really tricky.
As for why they’re M rated, I’m certain it’s because you can interact with corpses. The ragdolls in Halo never sleep like they do in other games, and even if they hit a resting position and “freeze” you can pop them right back out with a high caliber bullet, explosion, melee attack, or ballsack.
…I’ve been asking some higher ups in the CMT team (redundant acronym, I know) if it would be possible to “re-implement” the dismemberment feature, and so far it’s been the same old “it wasn’t there before so it won’t fit now” stuff.
ofc it will fit. I mean come on.