This is a tricky subject, since the seconds leading up to Zira's death can be interpreted in so many different ways.
Personally, it think she initially didn't, then reconsidered. When Kiara holds out her paw, you notice Zira makes a swipe at it...but I always saw the move as an attempt to attack and harm Kiara, not take her paw to save her.
But as several people above have mentioned, right as she's about to lose her grip, you see her eyes go wide as she realises her fate, and I believe, in that moment, she would have reconsidered, even if she knew it was far too late by then.
But as I said...tricky moment in the film, rather open to interpretation.