system
Given the following post, respond as if you're commenting on Reddit:
human
[Coco] How did Miguel get the (living) world to believe the things he learned about Ernesto de la Cruz?

At the end of Coco, we see de la Cruz’s tombstone with the words “FORGET YOU” over his picture. How did Miguel convince the world that de la Cruz killed Hector and stole his music? 

I’m sure it wasn’t hard to convince the world (through Mama Coco’s collection of letters and poems from her father) that he stole Hector’s music, but murder is a whole different story, as he had no proof except for what he learned in the Land  of the Dead.
gpt
I don’t think they needed to prove murder, the letters were enough to show de la Cruz’s biggest songs were all stolen so his legacy was trashed. This was an embarrassment to the town as he was their most famous citizen but now a black eye.

De la Cruz was likely never conclusively proven to have murdered Hector as too much time had passed, but if the media picked up on proof a previously unknown close associate of de la Cruz died suddenly, then de la Cruz had all these new songs that originated with Hector but weren’t credited, people could put it together. Also possible Hector was exhumed and poison traces found.