Carl is one of the most beloved characters on The Walking Dead, and his death was definitely a shock for fans. While the show is no stranger to bringing back dead characters—Merle Dixon, anyone?—Carl's death was pretty final. The midseason premiere of season 8 saw Carl bitten by a Walker, but instead of turning into one himself, he died. It's a hard choice to understand, but there are some explanations for why this happened.
In the comics, Carl survives to the end of the apocalypse, and is one of the first to help society evolve out of the world where walkers are king. He is the yin to Rick's yang, reminding his father to keep a tight grip on his humanity. Carl tries to convince Rick that the war with Negan must end, and that people can live in peace again.
When Rick's morals crumble in the wake of Lori's death, Carl steps up to be the voice of reason. When he is bitten by a Walker, Carl's last act of compassion is to let the creature kill him rather than letting it turn into a zombie. Despite losing an eye, he remains the same compassionate teen he always was, and he has a few parting words for his pop.
After the bite, Carl begins to lose his innocence, and he becomes a more stoic character who is less likely to fly into a fit of anger or get all emotional over anything. He spends his last day writing letters to the people he loved, and even wished Negan could have the same.