The use of force was definitely not justified in this circumstance. This is simply a revenge killing.
The use of force is allowable to prevent things from happening, not to avenge them. A person may use force if they fear that something bad is going to happen to them, to their property, or to another person. But that is about something that is going to happen, not about something that has already happened.
In the case that you are describing, there is no imminent harm that is being prevented. You can't say she's killing the guy so that he won't rape someone else because there is no immediate prospect of him doing so.
The use of force in self-defense is only allowable when there is a bad thing that is likely to happen in the immediate future.