The definition of 'murder' is about homicide, which means pre-meditated, thought of beforehand. If someone kills someone by accident (like in a bar fight by accident let's say), that is not pre-meditated and per God's law is classified as manslaughter. Under the Old Testament sanctuary cities were provided for those who did manslaughter, and the victim's family seeking revenge was not allowed to go after the person in those sanctuary cities. But in today's courts, judges and lawyers ruling from the bench treat manslaughter like murder putting various degrees on it, and a sanctuary city like a police free zone.
1st degree murder is a sin unto death. That's homicide, pre-meditated. Per the old covenant that always carried the death penalty. Today in the U.S., it depends on the laws of the state. Texas used to have a fast-track law for 1st degree murderers for execution.
Under the New Covenant, this is where many get confused, because Lord Jesus did NOT nail all of God's law on His cross. Jesus nailed the 'writing of ordinances' in the law on His cross, like Col.2 says. That is about the old covenant style worship, blood ordinances, do this, do that, eat this, don't eat that, etc. This is why Apostle Paul actually taught many of God's laws still being in effect under the New Covenant per 1 Timothy 1 and 1 Corinthians 6. Murder of course is one of them.
1 John 5:16
16 If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
KJV