If you want to speculate, he likely was the the reason for Garon's death or at least that's what I like to think. Following the wiki it generally is unclear how Garon died and even though he skills likely come from Anankos we can assume that at least a good portion of his physical abilities and his talent for wielding axes and magic was honed before his death. The difference being that while Sumeragi has basically become a full Vallite troop (meaning he is more or less a physical manisfestation of the man's spirit.) Garon however is basically a corpse being manipulated by Anankos from Valla, (Also why he becomes slime upon sitting on the throne.) There is no way of knowing exactly how it happened, but my personal idea is that Garon encountered the old Vallite King and the two fought with Garon winning but dying shortly after.
Note: Looked at other responses, it could be that the Vallite curse created a stronger connection to Anankos not to mention it could have simply been a matter of moving power. For example Anankos could've kept Sumeragi, Mikoto and Arete for the final battle but instead let's them pass on, this is likely not because Corrin slayed them but actually just Anankos releasing his grip as to gain power, similar to how he consumes Garon to get back power.