I'll start with focusing on the fun stuff. Ilya's reactions when Miyu was in that maid uniform was amazing
. I think we've managed to uncover one of her interests
. When that switch goes off Miyu's in pretty serious danger. Indeed Miyu and Ilya became closer friends...or so Illya claimed
.
But yeah things didn't go smoothly in this fight. This wasn't an opponent more powerful than Saber, but they got the first shot in and that alone was enough to make things very bad. One hit managed to throw off the whole group and could have lead to disaster.
But yeah, don't think anything bad of Miyu's actions here. Could see Ilya losing it and needed to do something. There's only one card left I think, but even one more fight might be too cruel to push onto Ilya right now. While obviously with tremendous abilities, this Ilya has grown up semi-normally. Thrown into these life or death struggles with others possibly about to die because of her actions is just too much right now. So Miyu took the road that would get Ilya out faster. I'm sure Miyu would rather take care of this last fight herself than force Ilya to go through any more suffering in these battles.