Since we have the law of karma then there should be no need for laws or prisons? The law of karma will issue out punishment automatically. So if you murder someone there will be equal consequences regardless. Is this correct?
Even in a world where the law of karma is real and in effect, one needs laws and punishment for a few reasons.
Karma doesn't "issue", and its effects are not "punishment"; in Buddhist view it's a blind process of cause and effect and the effects of karma are not to be known or speculated about. This is one of the four imponderables:
It's not known when the effect of karma comes to fruition, in what way, or if they come to fruition at all. If karma comes to fruition, one doesn't necessary realize: "This is because of my bad karma, I should change my ways."
Therefore, the concept of karma doesn't:
Which, in an ideal justice system, laws and prisons do.
"Since we have the law of karma then there should be no need for laws or prisons? The law of karma will issue out punishment automatically. So if you murder someone there will be equal consequences regardless. Is this correct?"
I essentially agree with your conclusion. The ones inflicting any sort of suffering will be "punished" by karma. This includes all who think they are inflicting "justice", another sort of suffering. No "justice" will take the place of one's karma. (This is how I understand the issue. Logic must fail eventually when speculating on these physical issues.) I also agree that, since we have, by Buddhist definitions, lived for countless lives, any attempt to enumerate or figure out anyone's karma will be unsuccessful. The cravings for all of these actions seems to be the driver so it is only through mindfulness, it seems, (and meditation) where one can change the basis for future events.
It is quite easy to have compassion for a victim (whose karma has caused his suffering). It is quite difficult to have compassion for those who are harming the victim (due to karma's result) also and those who think they must inflict justice (also due to karma's result). IF the idea of karma is true, then All suffering is from the result of previous actions.
However, as another answer notes, logic is only good up to a point. Beyond that point, only meditation can show one the truth of life (or their truth, perhaps). Speculation (even when using logic) is a product of the physical side of life and will, by definition, always fall short of the ultimate truth. So, all "answers" only "point" to the ultimate answer, which you (all of us) must find within you. (My "answer" so far, anyway.)
First of all we have to keep in mind that the karma is a activity which is based on reason and result theory (Pratītyasamutpāda). in karma reason is the cetanā( intention of the doing). Result of the doing which has done with cetanā(intention) depends on the nature of cetanā. if someone did something with good intention he will have good results and if someone did something with bad intention he will have bad results. But anyone except the Load Buddha cannot say the time taken to give the result. But all karma can be categorized into 4 types according to the time they will give the result,
Now let's consider them one by one.
1. Ditta dhamma wedaniya karma
Ditta dhamma wedaniya karma means the results will definitely be given in the same "Bhawaya"(Bhawaya refers to the time period from birth to die in one life). This is the situation where if someone can apply the karma as a low. But without the capability of determine the time of result-back it is useless to apply as a low.
2. Upapajja wedhaniya karma
Karma that will give the result definitely in next life. in other words, immediate next Bhawaya.
3. Aparapariya wedaniya karma
Karma that will give the result in between the third Bhawaya and before Nirvana call as Aparapariya wedaniya karma.
4. Ahosi Karma
Karma That will cancel due to the inability of giving the result is called as Ahosi karma.
So now anyone can decide the practical issue of using karma as a Low.
Prisons (and the laws that put people in them) serve two purposes:
Especially in the second case, we can see incarceration of dangerous people as an act of compassion towards potential future victims, to protect them from harm.
In an ideal world, society would be structured in such a way that individuals would be able to act according to their impulses, without the danger of causing harm to others. (Imagine if everyone had a 100% efficient robot bodyguard.) But until we reach such a state, laws act as a way to guide our behaviour and protect people from real danger.
Consider other laws pertaining to food safety (hygiene) and safety in the workplace. These prevent potential accidents which would harm not only the victims but also the people who created the danger (saving them from feelings of guilt and loss of karma). So these laws can be seen as compassionate towards both parties!
Because other laws serve their purpose and karma law is law of nature. So, to acquire our goals we establish our laws. With established laws we don't try to replace law of karma.
Degree of consequences is not established by the law of karma, only wholesomeness of them. Check this fine sutta AN 3.99.