superhero
kamen rider-well this is obvious
guyver-not the main, but others around him

fighting
busou renkin-epic antagonist, no dark setting
World Embryo-main character
ubel blatt-main
Kurozuka-main
3x3 eyes-main

horror
Dawn Tsumetai Te-main
Variante-main
Kurozuka-main
Tomie-main

tragedy
blood+-her minions (esp the transformation of one of her family member)
gunslinger girl-main

misc
Ahiru no Oujisama-its a shoujo, but I really want to put this here . rom-com so no dark setting at all...
narutaru-not really sure but i think there is some evidence of transformation near the end (dragon girl?)
Dorohedoro- main but the transformation was done before the story starts

sorry i didnt cross check with your lists but some of these should keep you busy for a while.

Kiseijuu - kinda, but it is still dark and reeeally great.

sorry about taking so long to reply to this(cause of uni and stuff), but parasyte, world embryo, ubel blatt and blood+ where almost exatcly what i was looking for and a great read.

figured i owed it to you guys to respond(agian sorry forgot to check it)

Tenjou Tenge would probably fit since Souichiro started as a student who likes to fight but it turns out that he's somewhat something else. well, just read it and see.

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Naoto ♥ Haine

Maybe you'd like Homunculus. The protagonist develops the ability to see homunculi after some head drilling.

It's kinda hard for me to find something comparable to Hellsing (my all time favourite manga )
But if it's just about the process of changing from human to another being I can list some stuff.
(If it was already suggested I'll just second it)
Dawn - Tsumetai Te (I liked the way he changes, can't compare it to Hellsing but it's a similar theme even though there are no vampires)
Immortal Regis (he turns into a zombie... It gets kinda dark later on - also there's a sequel)
Dance in the Vampire Bund (didn't like the anime though)
Mahou Tsukai Kurohime (it kinda fits but only later on.... can't say more though without spoiling a major plot twist of the series)
Sankarea (it's a comedy romance theme though ... no horror but about zombies)
Shikabane Hime (kinda fits only read the first couple chapters though)
Shingetsutan Tsukihime (bit of a stretch but it still fits)

Arago
Raiders
Monk!
Jack Frost - kinda
Mermaid Saga