Well the Flood are probably returning so I'd wager M for Mature. However, I don't see why the rating is important.Halo was always a rated M game because of blood and violence, plus the Flood. Despite having a light opening, Halo has always had that ONE moment where s*** gets real dark real quick and the gravity of the situation comes down like the Master Chief in the opening of Halo 3. It hits you like a ton of armor that 'oh man, this person is DEAD and probably suffered agonizing pain for HOURS before finally passing on'. In Halo: Combat Evolved it was Captain Keyes, in Halo 2 it was the Prophet of Mercy, in Halo 3 it was Miranda Keyes and Johnson (though their deaths were rather quick, Johnson was physically tortured for probably near an Earth day by Brutes), and in Halo 4 it was Cortana who was LITERALLY dying while Chief was slowly dying on the inside because of Cortana. The rating told you some serious business was going to go down and it was probably going to tear you a part emotionally.
None of that has anything to do with "Blood and Violence", especially since 2 of your examples are merely implied rather than shown.
I mean, in Halo 5 you see Jul get killed onscreen, so if just death fulfils it, then Halo 5 satisfy it too.
Also, Halo 5 right now is
currently rated for Blood and Violence. It's rated for more than Halo 3 is.
Pretty sure someone once said in the new Spiderman game, someone commits suicide on screen and it still gets away with a T rating.
People look way into this rating than necessary, until they find out that every Halo game was PEGI 16.