It's almost creepy how happy Frieren is, at the prospect of her disciple being able to kill her. It makes sense though - not just because of the past, but because she basically raised Fern, which parent wouldn't be happy when a child surpasses them?
This is definitely one thing. Her face lit up right after Fern suggested it at the end of the last episode.
After this episode, I think it also has to do with Flamme. It was Flamme's dream to make magic commonplace among humans. A human mage being able to kill her after so many years of her being essentially untouchable by humans means her teacher's dream is becoming more and more real.