Actually this does fit, he didn't have any previous training


A lot of the suggestion don't fit but some look promising.
What is killed doesn't have to be (but should) be human.
The killer is best when they are doing it out of necessity (e.g. Re:Birth - The Lunatic Taker the sister is both crazy and doing it out of necessity although she hasn't killed yet but still fits because she is still skilled at killing)

________________
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

Shiki received a lot of training.

It's very specifically mentioned...Somewhere.

Shut Hell

________________
Mad people either have no sense or too many extra senses...
On the net, men are men, women are men and children are the FBI. =D

still looking for more

________________
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

You could try something like Btooom! where a bunch of normal people are dropped into a kill or be killed situation. The protagonist does have video game experience but no actual martial arts or other physical training.

Arachnid? Only has a tiny tiny bit of training which was how to use the weapon for like a week that fitted into 1/2 a chapter

Re:Birth - The Lunatic Taker counts?

Maybe Bloody Junkie.
Here's the summary from manga fox.
"When Shouta finally finds a clue to his missing brother’s disappearance, he mysteriously wakes up in a room he’s never seen before.
The events of this room are broadcast online, and the only link with the outside is voice chat with the viewers of the broadcast.
In the midst of this extreme environment, a battle royale style murder game begins!

I'm not sure if he's skilled at killing or not because it just came out in 2011 but the protagonist is placed in a situation where he has to kill in a battle royale like game. Maybe later on in the series he will become skilled but so far there's only been one chapter.

Kyou kara Hitman fits the bill perfectly. It's also very funny (with ecchi )

Trace one of the main char is just a plain salary man that develop some mutations...

lots of good examples that fit too keep them coming

________________
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

The End

________________

How about real game, it’s a great manga by okushou though not much people know about it it’s still really nice to read. At the time I’m posting this there are 76 chapters. Truly recommend it.

[spoiler]It’s kind of a twist at chapter twenty-seven so yeah where the main character is exposed kinda then it starts diving into his dark past.

Arachnid