Yes this is an absolute law.
Our experience in war has taught us that humans are not well suited to killing. In some cases veterans of war return changed fundamentally after killing for their country. Obviously killing is not in our nature and if left unassaulted is something that will not happen. God law seems to fit nicely with our own instincts in this case so it should be a naturally easy command to follow and in most cases is.
Nothing like the breaking of this law has been so large a factor in helping us learn of good and evil. Killing is the most unforgiving act in term of the victim.
God prefers that we forgive all trespasses. How can one forgive if one is dead. The soul can and does but it leaves lessons unlearned and experiences unfelt by the victim and may force a resurrection for this soul. I base this on a biblical passage that place the age of man at 120.
I confess that my views, on resurrection, at this point in time are in flux and confirmation of this position would be appreciated.
Regards
DL