I'm fairly certain Serie wants to die, or I think the person that wants to kill her is one of her past students, cause she says that some of them end up doing things completely unexpected and surpassing her expectations, and she then says "can you visualize a future with me dying", I think this is hinting towards that one of her past students could visualize something others could not, and that was Series death and she's excited as to how that's gonna play out
Comment by Yourmom on Sousou no Frieren - Chapter 140
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@Yourmom 
@Yourmom maybe it is not that she wants to die, she wants the mages of this era to be able to surpass her. She clearly see that influence of Flamme on the Empire to dwindle but she might be hopping it is for the better.