Many battle shonen protagonists are bound by a strict moral code, which sets them apart from the antagonists they face throughout their journey. Some may adhere to a series of stringent principles prioritizing honor above all else, or remain staunchly devoted to their allies and loved ones no matter the cost. This is often a point that endears them to those they encounter, cementing their image as the "hero" of the narrative.
One of the most commonly observed moral dilemmas that most shonen protagonists face is the act of taking lives. There are examples on both sides of the spectrum here, from those who do not hesitate to kill, to those who refuse to do it under any condition. Regardless of whether the act of killing serves a purpose or not, this latter class of individuals see it as a cardinal sin to take life. A number of protagonists from major shonen battle manga and anime have stuck to this code, with this trait becoming an integral aspect of their character.
6 Izuku Midoriya
As the successor to All Might, who was once known as the Symbol of Peace in My Hero Academia, Izuku, who goes by Deku as his hero name, is a compassionate and selfless person to his core. Never hesitating to sacrifice life or limb in the service of others, he is completely devoted to playing the role of a hero, and always stands up for those too weak to fight for themselves.
He is also quite averse to taking lives, and has not accumulated a single kill in all the time he has spent as a Hero in training at U.A. High School. Deku's kindness and optimism are some of his best qualities, and his goal has always been to help and save those in need, including his opponents.
5 Monkey D. Luffy
Despite his brash and aggressive nature, the Captain of the Straw Hat Pirates has actually never killed even one of the numerous opponents he has faced over the course of his travels through the world of One Piece. More often than not, he either sends his adversaries flying away, or beats them into a state of near death, where they are knocked out and completely immobilized.
He will also never leave anyone to die, as seen in how he treated Rob Lucci and Kaku on Egghead Island when the Seraphim attacked them. Luffy's compassion is rare in the pirate world, where cutthroat tactics and betrayal are the norm among those who sail the seas, and this is one of the main reasons why he has such an impact on those he defeats.
4 Edward Elric
The youngest State Alchemist in Amestris' history, Edward Elric's quest to restore his brother's body as well as his own after a failed attempt at resurrecting their deceased mother forms the emotional core of Fullmetal Alchemist. Although his immaturity, stubbornness, pettiness, and cynicism may dominate over his better traits, Ed is staunchly against the act of killing or even using a Philosopher's Stone to perform alchemy — as they are created by sacrificing human lives.
He did not kill any of his opponents through the course of the series, which aligns with his deep respect for human life. Ed always wants to do things the right way, and will never sacrifice his morals for victory, which is why he is such a great protagonist.
3 Shigeo Kageyama
Even though he is easily one of, if not the most powerful esper in the world, Shigeo Kageyama, affectionately known as Mob, is a kind-hearted soul at his core. His compassion, humility, and his willingness to forgive those who challenge him, stem from his naive and optimistic worldview. While this does endear him to many in Mob Psycho 100, he is also prone to being taken advantage of by those with ulterior motives, the most obvious of them being his boss Reigen Arataka.
He is also very opposed to killing his enemies, and this can most notably be observed in how he treated Toichiro Suzuki, the leader of Claw, when the latter was about to self-destruct from releasing too much psychic energy. Instead of abandoning him, Mob extended a hand of friendship and attempted to take in some of Toichiro's energy, which helped contain the explosion and made Toichiro finally see the error of his ways and change.
2 Josuke Higashikata
The main protagonist of Jojo's Bizarre Adventure: Diamond is Unbreakable, Josuke Higashikata's empathy and kindness can often go unnoticed in the face of his delinquent nature and his tendency to express his emotions freely. While his Stand has the power to heal injuries and put things back as they were, he does not have the ability to reverse death, which is why he views life as something irreplaceable, to be treasured and protected no matter the cost.
He does not go out of his way to kill anyone directly, and even spared Yoshikage Kira after he was defeated, until an ambulance arrived on the scene and ran over the serial killer. Josuke also risked great peril to his own body while healing a seriously injured Okuyasu Nijimura, even though the latter was attempting to kill him earlier, which highlights his desire to save anyone he can from the finality of death.
1 Naruto Uzumaki
It is no real secret that Naruto Uzumaki's ability to turn his enemies into his allies is one of his greatest strengths, and there are a number of occasions where he succeeds in making his opponents repent their misdeeds and see the error of their ways. Refusing to go back on his word or give up even when facing impossible odds, Naruto's tenacity and willpower are only matched by his compassion, kindness, and desire to bring peace to the shinobi world in Naruto.
As testament to his desire to end the cycle of hatred among ninja from all nations, Naruto refuses to kill like his peers and mentors, believing that such an act would only lead to him become part of the cycle of pain and hatred that has afflicted ninja throughout the ages.