Murder, being the intentional killing of another person, cannot be justified.  There are however, numerous justifications for killing another person. 

For example, if I were to kill someone because I didn't like their ideology and thought they might do something bad to someone, that would be murder (I don't like them, I intend to kill them, and then I do the act). 

If I saw someone about to stab another person in the street and I believed that there was reasonable grounds to fear for the persons life, if there was no other way to stop the stabbing and I then hit the person with the knife with my car to stop him stabbing the other person, and the person with the knife dies from being run over, I wouldn't be charged with murder, it would be manslaughter as I didn't intend to kill him.

 Legislation usually provides for the use of force that is likely to kill or seriously harm a person, so long as such force is used by law enforcement agencies or other selected individuals in certain circumstances, such as a police officer attempting to arrest a person who has committed an offence punishable by life imprisonment, who has taken flight to avoid arrest, so long as the police officer takes reasonable steps to try and stop the person.  A doctor performing an abortion would also be covered as it is a legal medical procedure.

 Also, in war time situations, many people kill other people and it is not considered murder, again because the element of intent is not involved.  A soldier may intend to kill the enemy, but the soldier doesn't intend to kill a specific person because of any reasons or malice.

There are many ways to kill a person, but for murder there has to be an intent, either specific (I'm going to kill this person for this reason......), or able to be proved through actions (I don't say I'm going to kill this person but I'll just set up an event or series of events that target this specific person and wait for them to walk into my kill device).  I do not believe that murder is justified at all.