Wait, what?
Ok, give me a second to collect my thoughts and get these pesky Ree off my back. They're distracting.
*shoves the Ree into an air vent with orders to pester Vael*
Hmm, you want to make Voldie evil, while making him a father. I would say give him the excuse to himself that his daughter has some use, while she tries to redeem him. Since she is his daughter, you can allow him moments of niceness. She is his daughter, after all, so she gets some leeway. Since she was shaped, in part, by Dumbledore, it is to be expected that she would try to turn him to the light side.
You can have him show her his capacity for cruelty, along with periodic debates on various moralities. You can also show his and Harry's conflict as one of mutually opposing interests, at first. If she has a good connection to Harry, you can have him consider that as a means of letting her into the fold. Since she is trying to show that he can be saved from himself, you have a lot of options here.
The fact is, you can portray him as still being somewhat human, just a bad one, who somehow has a soft spot for his only child. The conflict between a parent who believes himself beyond such things, or just can't comprehend it, and a child who wants her father to actually be so, gives you a lot of room to be creative. You can make him a cruel bastard at first, and then have him soften up as Ashlee works on him. He can still be an evil bastard for a long time, especially as he has a reputation to maintain, which could slowly wear away until he hits a crossroads: his daughter, or power.
That could give you a good way to redeem him, even if his redemption comes from him
letting Harry kill him, and him apologizing to both of them in the end.