In general, it is morally accepted that people have a right to defend themselves if they are unjustly attacked or threatened. There are limits off course but in certain cases violence is accepted as a means of self defence.
If the right to self-defense gives someone the liberty to protect himself then the person can choose not to and instead become a victim. Whatever he reason might be for choosing so, if there is an obligation to self-defense, then failing to to defend himself would seem to be a moral failing. So there is there such an obligation ?
It seems that self-defence is a liberty but there appears to be controversy on the subject. Some believe that a man is forbidden to do something destructive to his life or to somehow not preserve it. So life is to be preserved and this would seem to impose an obligation of self-defense. Well then it could be argued that if someone feels threatened he could seek the protection of others.
However a person who relies on the protection of others expects those people to risk being hurt or killed to protect him. In the case of someone incapable of defending himself, it can be a morally acceptable situation (infants for example.) However, someone capable of defending himself but choosing not to do so in favor of expecting others to die for him would seem to be morally selfish. In this case, it would seem that people have an obligation of self-defense.
Yet people do rely on others for self-defense. After all, we allow the police and military to handle armed threats. Because they are armed and trained for such tasks, it makes sense practically and morally to rely on them.
A person will not always be under the watchful protection of others however. And there is still the moral concern of expecting others to take risks for someone when that person is not willing to do the same for himself. A person must be willing to accept the obligation of self-defense and not rely entirely on others.
A person would then be obligated to be competent at self-defence or at least of protecting themselves, thus being able to at least act on the obligation of self-preservation with some chance of success.