Islamic ruling on self-defence

Protecting oneself and one’s honour, mind, wealth and
religion is a well-established basic principle in Islam.
These are the five essentials which are well known to Muslims. A
person has to defend himself; it is not permissible for him to consume that
which will harm him, and it is not permissible for him to allow anyone to
harm him. If a person or a vicious animal etc attacks him, he has to defend
himself, or his family or his property, and if he is killed he is counted as
a shaheed (martyr), and the killer will be in Hell.  

If the harm that will result from this aggression is little
and he gives up defending himself for the sake of Allah, then undoubtedly
Allah will compensate him for that, so long as this does not cause any
increase in the wrongdoing against him or anyone else.