Yes, there are likely to be such fanfictions. The 'Harry Potter' universe has a vast and creative fanfiction community. Some fans might come up with the idea of Harry Potter being a Muggle assassin as a way to explore different storylines and character developments outside of the original canon. It could involve Harry using his magical skills in a Muggle - centered, assassin - like role, perhaps to protect Muggles from dark magic threats in a more covert way.
Perhaps an international wizard who has been trained in both magic and martial arts in some far - flung magical school. This wizard - assassin has his own code of ethics and when he meets Harry, he decides to take him under his wing, believing that Harry can be a force for good in a different way than the traditional Hogwarts - trained wizards.
It could be a rogue Death Eater who has mastered a new form of invisibility spell. This Death Eater might be seeking revenge on those who opposed Voldemort.
No. Harry Potter is mainly known for its own magical world and themes of friendship, good vs. evil, etc. Turning Harry Potter into an assassin fanfiction goes against the original spirit of the series and is likely not popular among the mainstream Harry Potter fans.
It could be a former Auror who has turned to more shadowy ways of dealing with threats. After seeing the inefficiency of the Ministry's methods in some cases, they decide to take matters into their own hands and become a sort of assassin to target dark wizards.
Perhaps in a fanfiction, Voldemort has made an alliance with a powerful villain from the Avengers' world and an ancient order of Assassins gone bad. Harry Potter, along with some of his Hogwarts friends, has to convince the Avengers to help them. They would then combine their different abilities. For example, Hermione could use her vast knowledge to figure out the weaknesses of their enemies, while Captain America could lead the group in battle strategy, and the Assassin could provide crucial intel on the secret workings of the enemy alliance.
One possible plotline could be Harry being secretly trained by an unknown figure to become an assassin against the dark forces. He would have to hide this new identity from his friends at Hogwarts.
It depends on your taste. If you like the idea of Harry Potter in a completely different and edgier role like an assassin, it could be interesting. However, some purist fans might not like this deviation from the original concept of the series.
One idea could be a story where a Sith Assassin somehow gets transported to the Harry Potter universe. Maybe they initially see it as a strange and weak world compared to their own, but then they discover the unique magic there. They could try to use their Sith powers to dominate, but find that the magic of Hogwarts and its defenders are not so easy to overcome.
In these fanfictions, the assassin brothers usually have a strong sense of loyalty to each other. They bring their own code of honor from their assassin background into the Harry Potter universe. For example, they might not use underhanded magic but rely on their physical and mental training. Also, they tend to be very observant, which helps them notice things that others in the wizarding world might overlook. This allows them to get involved in various adventures and mysteries in Hogwarts and beyond.