Looking for a manga like this (anime, too). The point is that the assassin hero(es) shouldn't be painted all black as a total anti-hero.

Even though it doesn't have the best art, Kyou Kara Hitman is pretty good and I personally enjoy it.

Maybe Kimi no Knife?

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This is an awesome series about assassins but it only has 2 chapters at the moment.

Akame ga Kiru!

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Perhaps you'd like Number. It is pretty slashy though...

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“All the kids have always known,
that the emperor wears no clothes,
but to bow down to him anyway,
is better than to be alone.â€

-Arcade Fire

Assassin's killing bad guys....Young Gun Carnaval comes to mind, but I didn't really think much of it - you might like it though.

There's Triage X, which takes the over-the-top action movie approach to the idea.

code:breaker

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Take the Magic: The Gathering 'What Color Are You?' Quiz.

code:breaker was the first thing that came to mind
maybe ichi the killer, that's a bit of a stretch though, and I didn't really like it.....
luck stealer, it's actually pretty hard to see him as a bad anti-hero.
Sorry, only code:breaker comes really close.

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"I don't hate people, I just feel better when they're not around."

Bum bum be dum bum bum be dum dum

"Don't you fly?"

Darker than Black?