I just think it's a case of he went insane after living too long and he needed to be put down. And she's afraid she might be reaching that point so she wants another Spirit contractor to kill her before she does something bad.The villainess reveals herself? Mmmm...
Seeing how this series loves to come with more tragic characters, It would not surprise me if we end up in an scenario with "there can only be one" rule for immortals... and she is actually wishing for another to become a spirit contractor so that they "break" her.
Since Lumis introduction she has been playing it up in a very melodramatic way, so it fits her perfectly.Kind of obvious, but the sense of drama is much better captured here than the LN. The artist made this exposition much more impactful.
In the LN, the readers only get to know what Lumi said, that she broke/destroyed the founder, but here in the manga, we get to see how the founder was destroyed in that final page. So, nice touch really.