I must say that self-defense or harming someone who harmed a loved one are totally different things.
If someone's child is being harrassed and its life is in danger and you kill the criminal to save your child, I classify this as self defense and justified.
If your child has been killed and only later you happen to encounter the killer... well, if you do take justice in your own hands, you deserve jail as much as he does. This may sound cruel but I cannot understand the eye for an eye method, even if it is a loved one. If someone would murder someone I love, I wouldn't even want to look in the mirror if I'd harm the killer ; in the name of justice one does not become the thing he claims to detest. To let justice be done is up to a court, and nobody else.
In Belgium there is a courtcase about pedophilia and murder right now and some people say it would be justified to let the relatives of the late children kill the murderer. I think this thought is pretty sick, it'd be revenge and that can never be justice. It is a different situation when your life is at risk or when you can save the life of a loved one so I would say to harm the criminal is justified obviously when the life of the child could be saved, but to harm the killer months after the child has been murdered does not solve anything.
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Do what Thou wilt shal be the whole of the Law.
Love is the Law, Love under Will. And...
every man and every woman is a star
(excerpt from The Book of the Law - Aleister Crowley)
"Od lo avda tikvateinu" (excerpt from the Israeli hymn)