Just So I’m clear, Evelynn didn’t kill...........
Vayne during their first encounter because she didn’t want to for whatever reason but because she couldn’t as vayne at that point wouldn’t succumb to any seductions Evelynn would dish out so she wouldn’t feel agony’s Embrace.
I’m not underestimating the power of runeterra demonic temptations, in fact I’d hazard a guess that had vayne been with her parents that night she to would have given in to Evelynns seduction and died with them however Vayne saw the demoness for what she was so her lure was useless
Which makes me wonder if Evelynn is not able to hurt anyone (even in self defense) so long as they resist her temptations. If true then that’s a useful fact for vayne when she eventually finds Evelynn