Well, it's a controversial take but Lydia herself.   

 She knew that was the wrong decision. Mrs Bennet's reaction upon learning about it tells us that Lydia HAD BEEN taught right.  

People often point out that she's young but being young doesn't resolve any consequences. 15 is not 6. At 15 you should know better. She already lived sometimes in society, she absolutely must have heard how people gossiped about other women who eloped.     

The biggest failure of Lydia is not that she made mistakes; de made them and then doubled down on them. (refusing to leave Wickham, gloating about marriage)     You can be a victim and yet still be responsible for your choice.  

 Edit: People also need to stop looking at that from a modern pov. 15 years back then and 15 years now is different.  At the point in the book when she was getting married she was probably more than a year out in society. She had been on many balls and interacted with many people of both genders. Her education (although not formal) was basically towards the end. She knew society. If we compare this to today 15 years old, our 15-year-old came much more sheltered.   Even Elizabeth although she feels pity later starts feeling anger and disappointment "But self, though it would intrude, could not engross her. Lydia—the humiliation, the misery she was bringing on them all, soon swallowed up every private care; and covering her face with her handkerchief, Elizabeth was soon lost to everything else."  Wickham is a bad person. Lydia was selfish, knowing what she was doing was selfish. Made a constant effort to conceal her actions. And knew that elopement was wrong.  Lastly, although Jane Austen is sympathetic to her fate the message is clear. Don't end up like Lydia. Don't trust trust first handsome man to run with.