Note: I will not attempt to argue for or against the actual existence of a God, rather the interaction between a God idea and a human need.
Arguably, one of the most destructive actions a human psyche can endure is taking a human life.
Whether the value lost is implied spiritually, or through family values, or through subtracted potential world-value (wherein the dead may have contributed to human altruistic worth) - is not important to narrow down. The point is there is value in life.
When you remove that value one's core is fundamentally altered and often corrupted. All orderers (those that command some to take the lives of others) use a "driving force" to attempt to allay the apprehension of this corruption of those about to take lives. Driving forces include religion, revenge, safety, freedom, law, or any number of ideologies. These are often effective in the short term - and sometimes are used to light a fire under those ordered. This fire drives them to the goal, but with time and introspection, that fire is often diminished. This leaves the life taker hollow and often morbid.
No one can remove or forgive what has been done. Not the victims family, not your family, not your community, not your government, no one.
But maybe God?
God, as the creator and ultimate end is the only one who can possibly correct the damaged human psyche. The need to be forgiven is so great in some that daring to deny their forgiveness (or Forgiver) denies any of their returned humanity.
This is an ultimate need in some life takers; to be forgiven. To be denied this threatens self destruction, or at worst, seeking absolution through external destructive action (terrorism or religious zealotry [though this would probably occur through an orderer as mentioned earlier]).
Life takers need God - or to thoroughly believe in the original driving force. Life takers with neither of these beliefs are psychopaths who do not see inherent value in life.