Guts for 8-A. Killed 100 men in a single battle, and routinely killed dozens per battle as a mercenary. Killed dozens of Apostles during the Eclipse, and would go on to kill dozens more as the Black Swordsman, along with hundreds of pseudo-Apostles. Has killed 1,000 supernatural beings at the minimum, as this is the required number to give Dragon Slayer the properties it has, and has at least killed hundreds of mundane beings, likely thousands.
Eren Yeager and Zeke Yeager for 8-C. Eren and Zeke orchestrated the raid on Liberio which caused hundreds to thousands of deaths, Zeke also wiped out the vast majority of the Survey Corps during the Battle of Shiganshina, thus single-handedly killing dozens to hundreds.
Annie Leonhart and Reiner Braun for 9-A. Were 2 of the 3 Marleyan Warriors responsible for breaching Wall Maria, which directly resulted in hundreds to thousands of deaths alone, and indirectly resulted in hundreds of thousands of deaths later on due to food shortages and purposeful culling of the displaced population.
Bertolt Hoover for 8-B. Same reasons as Annie and Reiner.
Eren Yeager, Ymir Fritz, and Armin Arlert for High 7-C (and also Low 7-B for Eren and Ymir) Eren orchestrated the aforementioned Liberio hijinks, caused thousands of deaths, later started the Rumbling and is currently committing genocide against the entire earth, has killed millions already. Armin was involved in the raid on Liberio, and let off his nuke-tier Titan transformation in an urban area, soooooo yeah. Ymir Fritz was the main work horse of the Eldian Empire during its starting years, and stomped country after country for them. Her Titan powers were passed to the ruling Fritz family who continued to abuse it, and by the end of the Eldian Empire they had caused in total tens of billions of deaths.