Murder has always been a taboo subject since it is the act of taking the life of another human being. Truly, it is a bad thing, but there are times when the act of murder is the most moral thing to do in certain situations. Humans have realized this and act accordingly but the moral stencil brought by the god of Moses known as the Ten Commandments (ironically) prevents us from making these moral decisions based on our own judgment.
Euthanasia is justifiable. Comfort is always preferrable to pain and for extreme pain and suffering, one will need extreme comfort, even if it means death. The only real decision to make is whether the sufferer is past his/her suffering threshold that euthanasia would be the only escape. Take Terri Schiavo for example. In that situation, the only two choices are more suffering for her and her loved ones... or euthanasia.
Revenge is a bit tricky but it still can be justifiable. Some revenge murderers have been set free while others were punished. It all depends on the gravity of the situation. In revenge, the murderer actually prevents further atrocities by killing the perpetrator.
Survival is justifiable. Self-defense is the most common example of this. But there are also life-or-death choices when limited resources are at stake; the death of some will ensure the survival of the rest and inversely, choosing for everyone to live will make everyone suffer or (ironically) die.
Laws are the foundation of society but there will be situations where one should decide for himself. With a critical and compassionate mind, humans can make the right decision when faced with this predicament.
