Minhaj al Muslim By Sh Tariq Appleby pt: 5

 Killing people is bad (that's a general consensus) and would definitely count as a sin but in self-defense or someone breaking into your house and you defend yourself and the result that is that the aggressor killed then it is not a sin.

It is a big sin to kill another believer.

Blood Money is what is money given by the family of the murderer as compensation for the murder to the victim's family (it is not the person back from the dead but it is compensation nonetheless).

Intentional Murder

Murder with the intention of killing, meaning you are killing that person with items such as weapons or with poisons, etc. It is all bad.

You cannot kill a pregnant mother you have to wait for the baby to be born and then when the baby is born, and then the mother will be killed.

If you want to kill someone or execute someone, you should not cause excessive pain to the victim of the execution, it should be quick and as painless as possible, as decapitation is the main form of killing as it is instantaneous.

We want to hurt that person but no intention is not murder

you are not sinful but it is recommended to give blood money.

Murder by Mistake

If you kill someone in a car accident, or you drop something on someone's head and kill them it is not your fault, or a knife gets loose and it hits a person and they die you are also not at fault as it was a mistake, or if you are hunting with guns and a person walks in front of a shotgun slug then it is not your fault either. (It is recommended to pay blood money though).

Retaliation

Retaliation is when you, yourself retaliate against the murderer.
First, you need to know that the murderer is of sound mind (meaning being sane), a child before puberty, and a sleeping person. Those are the types of people you cannot retaliate to. You also cannot kill your children.

If you want blood money, you cannot retaliate. You cannot kill anyone else in retaliation other than the murderer. 

Retribution

If you chop off someone's finger, then when you are caught your finger would be chopped off. if that was used, it would be very bad, even Saudi Arabia doesn't follow this specific law because it is too cruel. f you chop off someone's finger, then when you are caught your finger would be chopped off. if that was used, it would be very bad, even Saudi Arabia doesn't follow this specific law because it is too cruel. 

If the person who chopped off the finger then you must chop yours off, but if it difficult to replicate the problem says a concussion it is hard to replicate so you don't have to do that.