Will the sleeping angler comeback if he died before moving in?
He'll respawn again on his designated house or on the beach.
As for the death message, it is most likely due to the following reason as stated in the wiki:
He also has a unique death message: Instead of "[Angler name] was slain..." the message reads "[Angler name] has left!" This is likely due to rating concerns, as depictions of children being killed tend to result in an Adults Only rating from the ESRB, which might have limited the game's distribution.