If we look at the sequence of events, for once it doesn't look to me like Ranca even deliberately escalated to killing. Athryn was still on her feet and in fighting shape when Ranca took the opening offered. It proved to be an instantly lethal blow, but in the realm of "Oh well, sometimes that happens in duels."
Wh-what's going on here?[x] Threaten The Arslans. Ranca has just literally cut off the head of their leadership, pointedly accept this then inform the senior officers of the heavy cruisers that they are now under your command whilst she stands looking worryingly pleased with herself at your side.
In for a penny, in for a pound.