No-one loves their kids. The story starts with Harry miraculously surviving a murder attempt, and it’s revealed later that when his mother threw herself in the path of the spell, the force of her love for him caused it to rebound. But… the killing spell has been around for centuries. Surely in that time SOMEONE would have noticed that sacrificing yourself to protect a loved one causes rebounds?What this means is that when Lily sacrificed herself, she had no idea that the curse would bounce back to Voldemort, banishing him and so setting up the major plot of the series. To her, she was buying Harry the 4 seconds it would take Voldemort to cast another curse (slightly more if he's a slow speaker). There was nothing apparent that would stop him just killing Harry afterwards - no-one else was around to interrupt him, he wasn't going to run out of bullets, and it wasn't likely that he'd just change his mind.
I understand that faced with the imminent death of a child, a parent will do anything that has a chance of saving them, no matter how unlikely. But as far as Lily knew at the time, it was very unlikely.