um, i search for manga where the protagonist is cold-blooded. preferably male protagonist, the MC is so cold that whenever someone dies in front of his place, he's just looking at them like looking at bugs.
and not some protagonist where he's just cruel and likes blood (for example).
not also some MCs who's just screaming throughout the plot (like SnK, sorry, no offense intended. but Levi fits the requirements though he's not the MC)

manga that fits the requirement:
Goth
Ludwig Kakumei >> well......actually he's kind of laughing at the victims
Black Lagoon >> Balalaika, although she's not the MC
Noblesse
edit: added B.A.D. (SAKAKIBARA Sousou)

Julius Visconti is still the best captain.

[img]http://i.imgur.com/vUC343X.jpg[/img]

Maybe Samurai Deeper Kyo? 😕

Maybe One Outs, if sports manga are fine with you? Also Liar Game, although this one fits less.

Maybe this could help you: http://www.mangaupdates.com/showtopic.php?tid=35026

Mahouka koukou no rettousai. Slow developing, but fits the bill.

aletheia?
It's a Korean Webcomic though.

Vinland Saga?

[img]http://dragcave.net/image/4VQNA.gif[/img] [img]http://dragcave.net/image/VPOtG.gif[/img]

Elfen Lied
Dorohedoro
Hellsing
MW
Ningen Konchuuki

I second Dorohedoro and Hellsing.

Drifters
Until Death Do Us Part
Hunter x Hunter (some main characters more than others)
Berserk
The Breaker (first series only, referring to the older main character)
Blade of the Immortal

thank you all!!
i'll check one by one though i've read some manga (like Mahouka Koukou, SDKyo, and others)

Julius Visconti is still the best captain.

[img]http://i.imgur.com/vUC343X.jpg[/img]

Genocide from Red Eyes

[img]http://www.nodiatis.com/pub/18.jpg[/img]

Not really cold blooded, but maybe Joukyuu from Aiki
Really like the part where

he switched side to the enemy in the middle of the battle not many protagonist I could find like that.......
Wait...
I guess Im going to make another Looking For.... Thread.

Kami no Kodomo. Cant get link to work on tab.
This one should somewhat feel close to goth in your first post.
Its good if you don't mind the art.

Kami no Kodomo
Dont really want to second it. But maybe its what you looking for.
I've read it. I dont mind the art. Still confuse why its Josei.

I think the MC just got a very super ego and quite a shocking trauma of his childhood crush.

Code breaker-Oogami is a Code Breaker, one who "does not exist". He is a seemingly cold-blooded killer who follows the principle of "an eye for an eye", to "use evil against evil".

he was pretty much a real cold blooded mc..

Akumetsu-Akumetsu is the story of Shou, a vigilante that's out trying to straighten the corrupt Japanese Government through terrorism with his "One man, One kill" code of action.

cold blooded against the corrupt..

Quote from Great

Not really cold blooded, but maybe Joukyuu from Aiki
Really like the part where

he switched side to the enemy in the middle of the battle not many protagonist I could find like that.......
Wait...
I guess Im going to make another Looking For.... Thread.

yep, I really liked it when he switched side, cause I taught that it was for good, given with his personality. Kinda dissapointed though when he came back to the good side and when he's real motive for switching was revealed.

[color=violet]Tournament Round 1[/color]
[color=blue]Round 2[/color]
[img]http://i.imgur.com/CcwHreL.png[/img][img]http://i.imgur.com/aQ0E4sC.png[/img]