I went through many drafts of Impostor (seems to be a running theme in my writing process). In the original draft, Scotch actually died on the rocks (hahaha... oh) after being pushed off the cliff (ostensibly by Vee). We ended up keeping him alive because that would leave wayyy too much blood on Vee's hands. However, in order for the twist to play out, Vee still had to leave Scotch alone, not knowing whether he was dead or alive.
This was a major issue for me. It just didn't seem like something Vee (deep down, a good person who wouldn't purposely hurt anyone) would do. But then I started thinking about those really deep emotions--fear and love--and how they can cause people to do things they normally wouldn't. Vee was terrified that she would go to jail for her actions (which it turns out she didn't actually do). She also didn't want to abandon Mattie, since she was essentially mothering her sister at this point.
It also allowed me to toy with the theme presented in Nathaniel Hawthorne's Young Goodman Brown. Even the most pious people can do terrible things if their weakness is triggered. All of us are human, after all. Vee tries to do the right thing by calling the police, but it's too late. The damage has been done. We see the tension this dark secret causes among Vee, Mattie, Samantha, and Regina. All of them feel guilty. Samantha ends up coming apart at the seams. Although Vee finds out that she wasn't the one who actually pushed Scotch, she will never escape the knowledge that this dark side of her exists. The best she can do is try to make better decisions in the future.
To me, the best characters are the really complicated ones. The ones who have a million things going on in their heads all at once and are just fighting to do the best they can with what they've been given. They might make the wrong choices once in a while, but they never stop learning, growing. You might not always agree with their actions, but you can understand their point of view.
It's a necessary ingredient.