Alpha ruthlessly murdered ten characters, including the son of Carol and Ezekiel, Henry.
In The Walking Dead season 10, episode 12, titled "Walk With Us", Alpha tried to find Lydia (Cassady McClincy) so that she could prove her strength and eliminate her only remaining weakness. After capturing Lydia, Negan pretended to take Alpha to the cabin where he was holding her. After Alpha expressed her appreciation for everything Negan had contributed to the Whisperers, she stepped into the cabin, only to have her throat slit by Negan. At the end of the episode, Negan tossed the head to Carol, who remarked, "Took you long enough".
Having Negan kill Alpha instead of Carol was the best direction for The Walking Dead to take, even though comics readers likely saw the twist coming. This was how the comic book version of the story killed off Alpha, but some had wondered if the show would take a different approach by having someone like Lydia or Carol kill the character. The Walking Dead already missed its chance for Lydia to kill Alpha, but Carol being the one to do it still felt like a real possibility, since the show has spent time building up that rivalry.
Having Negan be the one to take out Alpha works for The Walking Dead because it adds something new to the comic story and provides resolution for two different character arcs at the same time. Killing the main villain of the season was a huge redemptive moment for Negan, who had been on this path for a while. Negan had other, smaller moments where he indicated that he had grown as a character, but killing Alpha felt like the culmination of everything Negan had gone through over the course of the last two seasons.
Also, Carol's master plan, which means that Carol masterminded Alpha's death. So The Walking Dead was able to simultaneously let Negan have his comic story and let Carol have her revenge. Just having Carol approach Negan and kill her out of revenge wouldn't have been as compelling. The Walking Dead has created an interesting dilemma, as it's hard to say how other characters will respond to Carol getting revenge on a murderer by unleashing one of their greatest enemies.