Everything on screen suggested that Cora was completely changed by regaining her heart.
Not everything on screen.
We saw, in the very same episode, that she was an evil, vindictive bitch even before she ripped out her own heart. Something she only did for the pursuit of power despite being deeply in love with Rumpelstiltskin. What makes you think she wouldn't do the same thing soon after all of this?
As someone else said, what Snow White did wasn't a noble way to go about it, but it was hardly an act of irredeemable evil. It was espionage, pure and simple. She couldn't have gotten close enough to place the heart into Cora in time, so she manipulated another force of pure evil to do the job.
Keep in mind that Regina had most definitely turned back to the dark side at that point, too. She had her chance. Several times.
And, yes, it will be incredibly hypocritical for her to act outraged by what happened considering everything she's done, including manipulating others into performing acts just as vile but
only for her personal pleasure/powergrabbing. Nevermind that the ploy Snow enlisted wasn't even close to what Cora herself tried to make her do with the exact same weapon.
Regina gets no sympathy, and Snow doesn't deserve to go to Hell for what she did. Guilt and regret over doing so? Sure, in spades, but only because she
is a good person.
If roles were reversed and Regina did the same thing to Snow, she'd just laugh musically and vanish in a puff of smoke. So why should
anyone feel bad for that evil twat?