Tatara is mad at Seidou's interference but he's then attacked by Takizawa's ukaku kagune and is then impaled. As he is dying, he thinks back to the Anteiku Raid and wonders if Arima got what he wanted by letting him carry away Kaneki.

Who kills Tatara?

One night, he was shot and killed by the Colorless King, which eventually resulted in HOMRA's manhunt for his murderer. Unaware to him, his death was staged by Nagare Hisui as part of "his game". Which makes Nagare the one responsible for Tatara's death.

Why did seidou kill Tatara?

He went so far as to strike down his Aogiri superior, Tatara, out of revenge for what he did to him during the Owl Suppression Operation, as a means to finally gain the approval of his ex-superior investigator Kousuke Houji, and to prove his ability to the CCG.