...Okay yeah zeo should die.
I'm not going to pretend like he can be redeemed, nor am I going to agree with Nanoha on this, unfortunately.
I get she tries to solve all conflict without murder, but there are scenarios where someone simply cannot be redeemed.
I don't particularly care that he "raised" Filly after, as the Mercure accumulation said, butchered her ENTIRE lineage... And it's even more evident in how an "outsider" can still sense Filly's consciousness whereas this "father" is immediately going for the killshot... he has absolutely NO redeeming qualities and I think keeping him alive for the sake of some "reconciliation" is extremely naive from the plot.
Also calling a make-believe father who killed her entire lineage and then the accumulation of her bloodline "equals" is simply one of the wrongest things Nanoha has ever said... I don't like it.