Also question is, does ACE really die in one piece?
As many fans would know, Ace died in the Marineford War protecting Luffy from being killed. It was quite a tragic death, in hindsight, it was quite avoidable. If Ace had not let himself be provoked and kept running, he would have survived and escaped, like the rest of the Whitebeard Pirates.
Correspondingly, why did they kill ace in one piece?
the Pirates succeeded in freeing Ace. But, he chose to sacrifice his life to save Luffy from Admiral Akainu. Ace's death was one of the biggest shocks in One Piece, because this series isn't really known for killing off people. But, he chose to sacrifice his life to save Luffy from Admiral Akainu.
One Piece Episode 491 Luffy after Ace's Death - YouTube.