While that is a nice sentiment, it is also naïve. Sure he could and is likely to live for decades, she fucked him up mentally during some of his most important years of life. He can't simply get over that (especially since it consumed most of his life already). There is a reason why child abuse is taken so seriously and viewed as heavily damaging.This was a good take, he realized that the girl in front of him is different person and cannot take his revenge on her.
This is good, as he can now slowly leave his revenge mission behind (since there is no point anymore) and focus on building a better life onwards.
"If you have capacity to do good..." she was a kid back then, and kids can do very evil things that comes to haunt them later in life if they are emotionally good person. That is called maturity as you grow, one you expirianced now by apologizing for hitting her despite what she has done to you previously.
"My life is ruined..." man you are 15-16 at best, average lifetime these days is around 80. Your life has just started and best years (20's and 30') didn't even start yet. If he starts focusing on building a good life now (especially with someone caring like she is now) by the time the two of them hit 30's they will be as happy as people who never expirianced bullying.
A quick aside but she isn't really caring, rather most of her actions stem from both the guilt of her past actions and the "kindness" he showed her. I am sure she won't be as cruel as she was as a kid but she isn't as altruistic as you think.