Most importantly, the family of the victims cannot forgive him for the actual murders;l they can only forgive him for what he did to each of them (pain and suffering over the loss of loved ones). Only the victims themselves and God may forgive him for what he did to the victims (which is why murder is the ultimate sin).
He does, however, appear to have met the requisite conditions for forgiveness:
-- he has admitted his crimes and accepted responsibility;
-- he has asked for forgiveness (I presume also to the victims who of course cannot respond); -- he has made amends as best as humanly possible and through his execution.
True forgiveness is yet to be judged.
The “forgiveness transaction” is not as simple as you conclude:
For example, many of us had to go through the process of forgiving someone who has passed on but it is too late. It is important to forgive on our side but how much sweeter it would be if it were complete.
Now in relation to God, it is also much sweeter if it is complete.