In the manga, Deku has not killed anyone. His character is centered around being a hero who uses his powers for good and avoiding lethal force. He believes in rehabilitating villains rather than eliminating them.
Kid Deku in fanfiction may also have his own self - discovery journey. He might start to realize the true meaning of being a hero isn't just about having a powerful quirk. Fanfic authors can explore his moral compass as a kid, and how he begins to understand concepts like justice and sacrifice. This is an important part of his character development that can be delved into deeply in fanfiction.
I'm not sure of the precise percentage. Different people might have different estimates. It's best to keep up with the manga's ongoing chapters and see for yourself how far Deku's story has progressed.
There's also the 'Kid Deku in a Different World' trope. In these fanfictions, Deku is transported to an alternate universe, either a completely different version of the My Hero Academia world or even a different fictional universe altogether. This allows for a lot of creativity as he has to adapt to new rules, new villains, and new allies while still being the determined kid Deku we know. It can lead to some really interesting crossovers and unique storylines.
Since we don't know the details of 'the kid has with the babysitter story', there are many possibilities. It could be that the babysitter read the kid an interesting story, and they had a cozy time sitting on the couch. Or the kid might have been a bit naughty and the babysitter had to be firm in setting some rules. Maybe they also went outside to play in the yard and discovered a little bug together, which led to an exciting exploration of nature.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
Training sessions could be a major factor in character development. Just like Naruto trained hard, his kid would too. During training, they could face difficulties and setbacks, which would teach them perseverance. They might also learn new jutsu and in the process, develop their own unique style, which would also shape their character.
It's possible that the kid is a result of some sort of time - travel or alternate reality situation. Maybe the kid is actually Naruto from a different timeline or dimension, and Naruto has to figure out how to deal with this strange situation while on his training trip.
Fanfictions should focus on positive, family - friendly and creative aspects rather than such improper content. So I won't discuss this particular one.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.