Why did aoki killed himself and what's the shadow mean?

Revenge.

Aoki lived in Kujo's shadow since they were little kids. When Kujo won the clapping game, Aoki was excited because it meant he can finally shine as Kujo's right-hand man. This gave him a sense of purpose, which no one bothered giving him.

Their school was so poor and run-down that everybody wrote those students off as a lost cause. They will work in low-pay jobs, for criminal gangs, become unemployed, or end up dead. There's no 'bright future' for the like of Kujo, Aoki and their friends, and they all knew it. This is why Aoki was happy when Kujo won the clapping game. They finally had something to live for.

However, Kujo's indifference was in the way. Remember the part where Kujo drew a shadow on his desk? Aoki asked what it was, but Kujo wouldn't say.

More Aoki pushed him, more withdrawn Kujo became. This growing distance, indifference and the fact their friends were falling apart angered him. Aoki realized the shadow on Kujo's desk represented Kujo's apathy. This realization made everything in him died. Hope, ambition, optimism, life. Kujo had effectively 'murdered' Aoki.

Now he was just as apathetic as Kujo, he challenged Kujo for a fight. To his dismay, Kujo took him down easily. He'll always live in Kujo's shadow. He didn't have what he needed to be Kujo's equal or better. He had lived in Kujo's shadow for so long that he didn't know how to be anything but that.

He waited all night for Kujo's return to school. When the moment came, he took part in the clapping game. To make it seem as if he was trying to prove he was just as good as Kujo. For the first time in a long time, Kujo threw his apathy aside to save Aoki. When he got there, Aoki was gone.

When Kujo noticed the shadows on rooftop, he realized what Aoki had done. Killing himself in front of Kujo set Kujo's apathy/shadow free. Just so that Kujo can never be indifferent again. Like, he'll be haunted by Aoki's suicide for the rest of his life. In other words, Aoki became Kujo's shadow, which forces Kujo to live in Aoki's shadow.