: As for killing off characters, who has 343 killed off needlessly (and before
: you say Black Team, I wouldn’t consider them memorable for any good
: reasons…)

The only major characters, at least in consideration to the stories they were killed in, to have died are actually few and far between.

000 Tragic Solitude (Hunters in the Dark) - Killed by the cybro Bobby.
Black Team (Initiation) - Were killed by advanced Promethean combat constructs.
Chyler Silva (Forward Unto Dawn) - Was a cadet soldier, got shot.
Cortana (Halo 4) - Saved Earth and the Master Chief. Succumbed to Rampancy.
Daisy-023 (Homecoming) - Needled.
Faber (Silentium) - Fell to his death. His own fault. Built Halos with heights.
Fal 'Chavamee (The Dueld) - Killed in a duel, also killed his opponent.
Ghost (Prototype) - Activated the nuclear self-destruct, saved retreating colonists.
Iona (Saint's Testimony) - Deactivated due to Rampancy.
Librarian (Silentium) - Likely killed by the activation of the Halo Array.
Merg Vol (Spartan Assault) - Simply got shot.
Mshak Moradi (Hunt the Truth) - Got killed exposing the tyrannical UEG's secrets.
Raia 'Mdama (The Thursday War) - Killed aboard frigate Cleansing Truth.
Randall-037 (Nightfall) - Manually activated nuke, destroyed a Halo ring fragment.
Rookie (New Blood) - Was a Marine, got shot in the head.
Ur-Didact (Initiation) - Still not even dead.

That's 19 characters in six years and 9 of which, despite being the main characters in their own stories (or in others), are far from memorable. I can't say that any of these above deaths were unreasonable.

Bungie's count...

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.

Haven't even got to the books yet and most of those deaths were all within a margin of a couple levels to within the same level.