There is no contradiction between forgiving the killer and still expecting justice. It’s not really for you or I to forgive the killer anyway, as the offence wasn’t really against us. But he should still face the power of the sword, both to uphold justice in his particular case and to serve as a deterrent against other evildoers. A failure to execute him not only forces taxpayers to pay for him for so long as he lives in prison, it emboldens others to follow in his path, with all the consequences that entails.
Unless he gets executed very quickly which would be unusual he’ll have a number of years to reflect on his dark deed.