Robert Maxwell wrote:
Given that Cora and Regina had designs on dominating and perhaps killing everyone in Storybrooke, Snow's actions could be viewed as self-defense. She certainly manipulated Regina into completing the scheme, but Regina's shown to be a deeply untrustworthy ally no matter what side she's on, so Snow was right to take the opportunity when she had it.
Defensible or not, Regina now has a truly legitimate reason to despise Snow. Killling Cora herself would have been one thing, but manipulating Regina to kill her own mother by appealing to her already-weakened sense of love?