To murder is to take the life of another with willful malice and forethought. Murder is not the same as self-defense or an accidental killing. Murder is not the same as imposing the death penalty on a person who has killed someone. This was a judgment established by God in Numbers 35. One of the first commandments given by God was "Thou shalt not kill".
Jesus said: ...thou shalt do no murder... (Matthew 19:18)
You should not be guilty of murder: But let none of you suffer as a murderer, or as a thief, or as an evildoer, or as a busybody in other men's matters. (I Peter 4:15)
The New Testament expands the meaning of murder to include hatred. If you hate other believers it is like being a murderer: Whosoever hateth his brother is a murderer; and ye know that no murderer hath eternal life abiding in him. (I John 3:15)