Whether or not someone is a fan of Hunter X Hunter or not, most would think Killua was a harmless young boyーthat That is until they discover he's coming from a line of assassins and has been trained to be a skilled one the moment he was born. Either way, this shows that even the most innocent person can be a cold-bloodedーor simply skilledーkiller.
However, this doesn't only extend to children and can go as far as having a harmless-looking adult, or non-human, harbor intense bloodlust within them. It would bode well to not judge these assassins on appearance and stay away from them in general.
10 Killua Zoldyck (Hunter X Hunter)
When he's not showing off his bloodlust, Killua is a normal boy who enjoys eating sweets, playing with his yo-yo, and skateboarding in the early chapters of Hunter X Hunter.
However, he comes from a highly-skilled assassin line and actually escaped from his home by wounding his mother and father to enter the Hunter Exam.
9 Kenshin Himura (Ruroni Kenshin)
Simply put, Kenshin isn't the run-of-the-mill definition of an assassin. However, he used to be a skilled samurai who has killed many peopleーand because of this. He vowed never to kill another again.
Although he didn't look quite as kind as he did back when he was an assassin, Kenshin never looked as if he was evil or a killer in the first place.
8 Akatsuki (Log Horizon)
Akatsuki may appear cute and docile, but she's actually one of the most skilled assassins in the MMORPG the main characters are trapped within in Log Horizon.
Akatsuki can single-handedly attack an entire crew of people and bring them each fatal blows. Despite her adorable exterior, she isn't on to mess with at all.
7 Koro-sensei (Assassination Classroom)
Koro-sensei is the culprit who supposedly blew up the moon and is a yellow tentacle monster. Technically, he should be someone who is fearedーhowever. He also happens to have a permanent smile on his face and always puts his students' well-being first, despite them trying to kill him 24/7.
The thing about Koro-sensei is that he should appear frightening, but he isn't. This is because of his kind personality, which is further exposed at the end of the series. It was quite the tearjerker after learning everything about the story and the character.
6 Kurome (Akame Ga Kill!)
Akame may be Akame ga Kill's main heroine, but the one who truly appears innocent is her little sister, Kurome.
Despite being younger, she is a skilled assassin who wishes for her older sisters' death after her so-called betrayal. Sadistic and calculating, Kurome will have someone dead before they can scream bloody murder.
5 Ein (Phantom: Requiem For The Phantom)
Ein is the partner to Zwei, a kidnapped young man, to become an assassin to fight against others. At first appearing as a silent young woman, Ein knows how to use any and every kind of gun, grenade, knife, and even non-fatal item to her advantage.
She even knows how to shed her emotionless facade and act cheerfully to stop her opponents' breath, making her an adorable but deadly girl to cross paths within the end.
4 Train Heartnet (Black Cat)
Before Train became the free-spirited character he was, he was a cold-hearted assassin known as Chrono Numbers. He may not kill anymore, but he was known as the second or first most powerful of the bunchーnot to mention he has the number 13 as his number.
After he met a young Sweeper named Saya Minatsuki, however, he changed his entire outlook on life and stopped killing othersーbesides the loathing he has for the man who killed her, his former partner, Creed.
3 Hei (Darker Than Black)
Hei from Darker than Black is the epitome of don't judge a book y its cover. Normally, Hei goes by an alias and normally lives, happily, and acts kind-hearted towards his neighbors and strangers alike.
However, it turns out he's mainly expressionless and doesn't like to speak to othersーhe also acts as an assassin who hunts and kills other people with abilities and powers like him.
2 Nagisa Shiota (Assassination Classroom)
Nagisa may be the main character of Assassination Classroom, but he's barely noticeable as a character. However, that's what makes him the perfect assassin. After all, the perfect trait for an assassin to have is invisible to their opponents.
Because of him having no presence, no one knows where he's coming from or where he's going to attack. This is mentioned several times, as he also has immense bloodlust that even scares professional assassins to their core.
1 Reborn (Reborn!)
Reborn is literally in a baby's body, but he's actually a full-grown manーas well as the best assassin known in the Reborn universe. Although his powers are vast and far, there sealed as he isn't in his full grown-up form.
From the tales he tells Tsuna, he is a feared mafia member who has defeatedーand killedーmany over the years, making him the best of the best. However, from his small and adorable appearance, no one would know of the powers this baby has at his fingertips.