From The Hero In His Past - Silver Award: From The Hero In His Past

[MangaDex]

 

[LIXOUA]

so how did he not die when he killed himself

 

[Amiableentropy]

so how did he not die when he killed himself

The future "hero" did die. At the end, that's the past/present "hero" that escaped the tower after being influenced by the faint memories of his futureself that lingered after he(the future "hero") was gone.

 

[RiverSticks]

so how did he not die when he killed himself

Another theory:
Since he prevented his younger self from seeking a life of self-destructive vengeance, Future-mc no longer existed.

The past was changed for the better, so the terrible future never happened.

Assuming single timeline system, of course