: Jacob Keyes (Halo 1) - Flooded and punched.
: Sesa 'Refumee (Halo 2) - Assassinated by the Arbiter.
: Prophet of Regret (Halo 2) - Assassinated by the Chief.
: Prophet of Mercy (Halo 2) - Flooded.
: Tartarus (Halo 2) - Arbiter'ed.
: Miranda Keyes (Halo 3) - Shot in the back by the Prophet of Truth.
: Prophet of Truth (Halo 3) - Stabbed in the back by the Arbiter.
: 343 Guilty Spark (Halo 3) - Lasered by the Chief.
: Avery Johnson (Halo 3) - Lasered by Guilty Spark.
: Gravemind (Halo 3) - Eradicated on the replacement Alpha Halo.
: Auntie Dot (Reach) - Went down in a crash.
: Carter-A259 (Reach) - Forgot to eject from above crash.
: Kat-B320 (Reach) - Headshot. No Scope. Newb.
: Emile-A239 (Reach) - Arbiter'ed by some no-name Elite
: Jorge-052 (Reach) - Went down with the ship.
: SPARTAN-B312 (Reach) - One of those gruesome stabbed-a-million-times murders.

Context is important. Most of these deaths serve a larger dramatic purpose, and almost all of them take place after the affected character has fulfilled his/her narrative purpose.

Any death from Reach doesn't count IMO, because the whole point of that story is that Reach is screwed and everyone is going to die, so gratuitous deaths are necessary to tell that kind of story.

Halo 3 only sort of counts because it is the climactic end of the trilogy, so big character deaths are kind of expected. The big climax is when things go to hell and everyone plays their best cards, so some collateral damage is not only expected, but necessary, in order to raise the stakes.

That leaves Halo 1 and 2.

IMO, Keyes' purpose in Halo 1 was to die. He was written as an authority figure and a general badass specifically so that his death would have a greater impact. His narrative purpose was fulfilled in death, so it felt entirely necessary.

For me, Halo 2 is the only game that has an unnecessary death. Regret and Tartarus are boss battles, so Bungie set them up to die, but I feel like they could have gotten more mileage out of the Heretic Leader, and Mercy's death feels wasted. (Although it does serve to highlight Truth's back-stabbing nature, back when he was still a scheming political mastermind, before Halo 3's character re-write.)

That's just my opinion, of course.

Meanwhile, many of 343's deaths feel very sudden, like they happened before the character had a chance to fulfill their potential, and are questionable from a narrative perspective.