When Regina pulled Snows heart out she sad it had darkness in it, meaning Snow will one day be bad. Is that actually going to happen? Will Snow turn bad?

Snow made a mistake. But she didn't embrace the darkness, as long as Snow does the right thing from now on I don't think Snow will become bad.

In the show, it isn't as simple as a character being good or bad, a hero or a villain (one of the many things I like about it). Having a spot of darkness in her heart is a symbol of how Snow has done something wrong or villainous. It doesn't automatically mean that she's going to become a villain. It doesn't indicate anything of a character's future.