It is important to understand the difference between moral laws, which are immutable, and other laws in the Bible, often categorized as civil or ceremonial.
In modern legal theory, we have a similar distinction, which you may know from Legally Blonde: malum in se (that which is inherently wrong) and malum prohibitum (that which is wrong because a lawful authority prohibits it). Example: endangering the safety of others by driving too fast is wrong in itself, while breaking the speed limit is wrong because the town said so.
The former type (moral, and in se) doesn't depend on the existence of a government or system of laws and justice. The latter type certainly does, and the wrongness of those things 1) is changeable at the will of the governing authority, 2) applies only to those properly under the authority, and 3) expires when the governing structure expires.
Christians believe the ten commandments are moral law, whereas when we get into the ins and outs of the ancient Israel system of justice, described at length in Leviticus, eg, we are seeing one civilization's implementation of that moral law into civil law, coupled with religious law and ceremonies that symbolized and pointed to deeper realities.
When Jesus says he does not abolish but fulfills the law (and he means all of the law, not just some laws), he can't mean you are now free to murder. He means if you murdered someone, you may yet receive eternal life and not eternal death, because he can pay the penalty on your behalf. He means he lived a perfectly moral life and can impute that to you. That's fulfillment of moral law.
It also doesn't mean eating pork was immoral and now it's moral. That dietary restriction was a malum prohibitum component of the ceremonial cleanness symbolism that was meant to point the Israelites to the reality of what happens to a soul that consumes unholy things. It was never immoral, only prohibited. But the law is fulfilled in that the symbol is no longer required. Jesus institutes a new symbol that fixates on his own holiness and cleanness imputed to us as if by eating it (the Lord's Supper). The early Christian movement quickly incorporated non-Jews who had no heritage of living under the authority of the Israelite state, which was, by this time, already expired/expiring, many times over through successive conquests and occupations by various other governments. Neither the non-Jewish Christians nor the Jewish Christians needed to abide by those laws, though many did for a time, as it was an integral part of cultural and religious custom.
You're right, Jesus isn't saying "you are free to murder". But in every case we see Jesus prevent (or reverse: Luke 22:49–51) bloodshed, the bloodshed would have been retribution or punishment. Jesus is presented as reasonably anti-murder. Whether that holds in the case of capital punishment, I can't say.
Though, Jesus was subject to capital punishment, despite the original prosecutor (Pontius Pilate) claiming no lawful basis for the execution (Luke 22:66–23:25). This was presented as unjust, despite Jesus' insistence that it had to happen. This is a bit muddled as an analogy, but I see how some people might take "capital punishment is wrong" from it.
In modern legal theory, we have a similar distinction, which you may know from Legally Blonde: malum in se (that which is inherently wrong) and malum prohibitum (that which is wrong because a lawful authority prohibits it). Example: endangering the safety of others by driving too fast is wrong in itself, while breaking the speed limit is wrong because the town said so.
The former type (moral, and in se) doesn't depend on the existence of a government or system of laws and justice. The latter type certainly does, and the wrongness of those things 1) is changeable at the will of the governing authority, 2) applies only to those properly under the authority, and 3) expires when the governing structure expires.
Christians believe the ten commandments are moral law, whereas when we get into the ins and outs of the ancient Israel system of justice, described at length in Leviticus, eg, we are seeing one civilization's implementation of that moral law into civil law, coupled with religious law and ceremonies that symbolized and pointed to deeper realities.
When Jesus says he does not abolish but fulfills the law (and he means all of the law, not just some laws), he can't mean you are now free to murder. He means if you murdered someone, you may yet receive eternal life and not eternal death, because he can pay the penalty on your behalf. He means he lived a perfectly moral life and can impute that to you. That's fulfillment of moral law.
It also doesn't mean eating pork was immoral and now it's moral. That dietary restriction was a malum prohibitum component of the ceremonial cleanness symbolism that was meant to point the Israelites to the reality of what happens to a soul that consumes unholy things. It was never immoral, only prohibited. But the law is fulfilled in that the symbol is no longer required. Jesus institutes a new symbol that fixates on his own holiness and cleanness imputed to us as if by eating it (the Lord's Supper). The early Christian movement quickly incorporated non-Jews who had no heritage of living under the authority of the Israelite state, which was, by this time, already expired/expiring, many times over through successive conquests and occupations by various other governments. Neither the non-Jewish Christians nor the Jewish Christians needed to abide by those laws, though many did for a time, as it was an integral part of cultural and religious custom.