No, it's not. I'm saying that I can understand why she would react the way she did, not arguing whether or not it was the right thing to do. I'm just focusing on a minute detail and expanding on it, perhaps turning it into a bigger deal than it deserves to be.
The thing is, IIRC (I don't have #32 handy at the moment) she thought that Genocide would die by letting her drown, killing her passively. She just didn't want to perform the act of killing her with her own hands. It's very "I won't kill you, but I don't have to save you," a la Batman Begins. It has the effect of killing*, but saves WW from the burden of doing it directly.
*Theoretically, because Genocide and R'as don't exactly have to follow the rules.