I'm looking for a manga where someone who lived a normal life is put into a situation where he/she has to kill and turns out to be very skilled at it(killing that is).
NO being raised as an assassin or any previous training to kill or PREVIOUS martial arts training (figuring it out themselves is OK)
Reason to kill can be revenge, self-preservation, or just plain snapped.
Prefer it to be from said killer's perspective.
All genres are fine except Yaoi and Shoenen-ai.
Only examples I can think of are Phantom Requiem of Phantom, Guyver and Code Geass

All choices are made in fear,
What you fear more is the only difference.
Positive decisions are made in fear negative consequences,
Negitive decisions are made in fear of positive decisions not giving the outcome you want.
Embarrassment is the fear of people knowing your real thoughts.
Greed is the fear of having too little.

Demonblaze0's personal thoughts

shamo?
kids snap and kill his parents and hone his skill in prison with a deadly sensei? 🙄

= Stress is what we Live FOR =

/===CODE OF CONDUCT===~\
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Death Note?

Granted, he doesn't have to kill... but his ego is amazingly huge... You may like the anime for it (I enjoyed it).

1/2 Prince / 1/2 Prince (Novel) - but it's in a game, so it's not really that serious.

Highschool of the Dead - they're technically already dead, but ah well.

Tetsu no Senritsu

Haven't read Death Note but I seen the anime didn't like the ending.
Already reading Highschool of the Dead and 1/2 Prince and I really like them.
Tetsu no Senritsu is too old for my tastes.
And I get the feeling I won't like Shamo sounds too depressing
Looking more for dark or twisted instead of depressing and try to avoid manga with the "Rape" tag.
All them are close but I don't think any of them fit skilled killing with no trainig except Death Note_ as the initial situation allowed for easy killing (e.g. Highschool of the Dead the only skilled killers had previous training, 1/2 Prince the main character had played a similar game before don't know about Shamo_ though).

All choices are made in fear,
What you fear more is the only difference.
Positive decisions are made in fear negative consequences,
Negitive decisions are made in fear of positive decisions not giving the outcome you want.
Embarrassment is the fear of people knowing your real thoughts.
Greed is the fear of having too little.

Demonblaze0's personal thoughts

Gantz? I'm only 2 volumes in though, so I dunno how skilled the protagonists are at killing.

[img]http://i370.photobucket.com/albums/oo143/TurboPhoenix/FireflySig.jpg[/img]

maybe kimi no knife

Quote from demonblaze0

Haven't read Death Note but I seen the anime didn't like the ending.
Already reading Highschool of the Dead and 1/2 Prince and I really like them.
Tetsu no Senritsu is too old for my tastes.
And I get the feeling I won't like Shamo sounds too depressing
Looking more for dark or twisted instead of depressing and try to avoid manga with the "Rape" tag.
All them are close but I don't think any of them fit skilled killing with no trainig except Death Note_ as the initial situation allowed for easy killing (e.g. Highschool of the Dead the only skilled killers had previous training, 1/2 Prince the main character had played a similar game before don't know about Shamo_ though).

There's actually a more recent manga based on Tetsu no Senritsu, but I can't remember the name. I'll post here if I find it. ^^

Less Explicit
Involve killing monsters. Usually not as disturbing as the ones recommended below. May be off mark.

Blood+ - Schoolgirl discovers she has incredible destructive power when she is forced to kill a monster rampaging around the school.

Dawn - Tsumetai Te - Contracts a disease that renders him able to tear apart other infected individuals with his bare hands. To continue to get the medication necessary to keep the illness under control, he must cull the infected. If i remember correctly, he kills the first one out of self preservation then flips out because he thinks he's killed someone.

Explicit/Dark/Twisted
There stories may be too explicit. They do have what you're looking for, though. These stories mostly involve killing people.

Suicide Island - The main character is deserted on a remote island with many other multiple suicide attempters. There he discovers that he is a very good hunter. Very dark and twisted. I think there is some rape in it, though. It is very brief and the result of the lack of leadership and structure in the beginning.

Shingeki no Kyojin - The main character and his foster sister kill an intruder very skillfully for their young age. They grow up to become soldiers.

God's Child - Extremely disturbing. About the life of a sociopath that becomes a serial killer. Warning: explicit.

[img]https://lh5.googleusercontent.com/_YWoztpsbz1g/TdnBLYIyyFI/AAAAAAAAAQU/SQRRMGaFWfg/banner.jpg[/img]
Image from Gangsta -> Full Image Link http://www.k4.dion.ne.jp/~babyrock/page030.html

Maybe Tsukihime 🙂

[img]https://c1.staticflickr.com/1/753/22668593615_519ce54b81_o.png[/img]

Freesia

Quote from Avian3

Maybe Tsukihime 🙂

He had training.

Lots of it.

Some of the above do not fit this criteria:

Quote from demonblaze03

NO being raised as an assassin or any previous training to kill or PREVIOUS martial arts training (figuring it out themselves is OK)

Battle Royale

[img]http://valenth.com/lab/69359.png[/img]Feed Me!

what about hoshi no samidare - but his priority is to protect the princess but eventually he has to learn to kill the monster anyway....

Superior day
Duds Hunt
Ikigami
Judge (TONOGAI Yoshiki)
Anne Freaks
Dragonhead
Arms-- not sure. Haven't read this for a long time.
Higanjima

[img]http://img803.imageshack.us/img803/6167/avvy.png[/img]
Rape is imminent.