Who Killed Tanjiro?

Towards the end of the fight, the main antagonist of Demon Slayer, Kibutsuji Muzan, perishes. Moments before dying, he reaches for Tanjiro and transfers all of his blood into his body. Just before injecting blood into Tanjiro, he stopped breathing and was dead for a few seconds.

Who killed Tanjiro Demon Slayer?

Muzan is capable of overpowering Tanjiro still and eventually poisons him which weakens him and blinds him in one eye which leads to his death following Muzan's demise.

Why does Muzan kill Tanjiro?

As mentioned, it is revealed that Muzan's desire to destroy Tanjiro stemmed from his hatred towards his previous nemesis, Yoriichi Tsugikuni.