Logically, the loss of life will not be payed for equally. That is, no matter how long the sentence for the criminals, they still don't have to undergo the suffering they inflicted upon the victim.
For this
reason, I fail to see why any leniance or bias in favour of the criminal, should be observed.
The loss of life is
potentially an eternal loss for the victim, if the universe is atheist, and thus can never be replaced.
I would say there are two important factors that must be recognised;
1. To make sure the crime is not commited again, (which happens a lot because of leniant sentences).
2. To make sure that the family of the victim, is adequately served some form of justice, by detaining the criminals as long as is fair and is possible.
In this case, how could one argue that life in prison is unfair, when the criminals still get to live in a comfortable state?
There are starving people, millions, who don't get fed and housed, like criminals do.
These two factors must always have the most weight, IMHO.