<Snipped quote by DarkwolfX37>
No, there really are. Take Nagisa's "natural talent for assassination." I'm not totally finished with the second season (episode 20 or so), but even with some sort of explanation, it doesn't particularly make sense. Karma giving up the final blow in the fight also didn't make sense. He claimed it was because the groups wouldn't unify if he did, but that's not very realistic, nor does it fit in with Karma's personality. He also went in with a ton of skill, destroying two of the tentacles in his first day, but as the common trope of anime goes, quickly loses those skills when new characters join the fray. He's presented as one of the strongest out there and is for a few episodes, but he quickly becomes just another part of the cast.
Nagisa's talent is just a natural talent. Just like some people are naturally talented at sports, or naturally good at reading or math, he's good at assassination. He has a ton of latent killing intent, which is a weapon in and of itself in AC. He's also a very good and fast learner when it comes to skills related to assassination. Again, just like how someone might be a natural writer or natural athlete. It's not any different than any other natural talent.
Karma gave up the final blow because using a weapon that he only had access to because of chance when both of them had already given up weapons for their fight would be dishonorable and
would cause a rift in the class. The whole point of the fight was to remove the rift that was there, and Karma saw that Nagisa was even more determined than he was, so stealing the victory in a dishonorable way is out of the question because both it, like is stated, wouldn't unite the class, and because it's not something Karma would do. Remember, this fight wasn't just any fight, it was an all out fight with his best friend over something they both cared about. His decision to not use the knife is just an extension of his and Nagisa's decision to meet face to face for their final clash, rather than try to shoot the other when they appeared for it.
He didn't lose his skill, he lost his motivation to kill Korosensei. He risked his life to kill him and it failed, so he basically decided to just go with the flow instead of going all out constantly. He decided to become a part of the class instead of being on his own. His attempt when he jumped off the cliff was his all or nothing attack. This was a major change for Karma, and there are more major changes for him throughout the series. It's a sign of his growth as a person, not a loss of skill. Not to mention that Korosensei saved his life, so of course it's harder to go all out trying to kill him, especially since this is also the moment that he grows from a loner to a part of the group, which in itself is to take the assassination less seriously.
