They look so damn similar, from the style and color of their armor down to their behavioral traits. Seems like Bungie simply saw the success that Doom had over their Marathon series and thought to themselves “you know what our next game needs? A massively overpowered super soldier in green high tech armor who spouts sarcastic one liners in the face of an overwhelming alien invasion” and went to town with that concept. And it worked like a charm, they cranked out six massively successful games before id/Bethesda decided it was time to reboot the Doom franchise 2016.
Perhaps it was payback for them to give Doomguy the Crucible in the reboot games (which looks a lot like the energy sword from Halo), but I can’t help but think these two are essentially the same character.
About point 2: I would argue that you could convince any judge that these characters are sufficiently distinct in the legal sense. IRL, people might still confuse them, but that’s not the point here. Just look at some of the competing car designs to get a feel for what is legally distinct enough these days. You can come up with a cartoon character, car or a sci-fi warrior that copies over 90% from something else, and you can still get away with it.