Yeah, on second thought there is no need for this to be a Ritual unless it's going to be one anyway (high-level, high-duration...). I think that's the best approach.
I don't like that just adding magnitudes makes the undead more powerful. I'd prefer powerful undaed to have unique elements. Here is an example (rather dark) spell from one of my old characters:
Forging of the Hauweta
Rego Mentem (Corpus, Ignem), Ritual
R: Voice, D: Permanent (non-Hermetic), T: Special
This spell is cast on two persons that are burned together on the stake as part of the casting. The two must be connected mystically in some way - Dracul, the inventor of this spell, used it on mothers pregnant by the other's seed, casting this spell as a punishment for infidelity.
The spell binds the spirits as they leave the body, forcing them to remain in the burning bodies and obey the caster's commands. It also binds the bodies together, creating an ever-burning mesh of bodies and souls.
The resulting undead, called Hauweta, always refers to himself as "we", and speaks earily in two voices. It is in eternal agony, forever burning in unnatural fires, but is forced to obey the caster's commands even through its suffering. Hauweta are immensly strong, but fragile (Dracul wraps them in metal armor). In addition to their martial capability, magic resistance, and utter loyalty, a Hauweta can make others share its endless pain (R: Eye, D: Diameter, T: Individual; Me; cause immense, debilitating pain).
This is a matter of Magic Theroy 
As I see it, Hermetic magic cannot directly command an object to have volition. A ReCo spell controllong the body could move it around, but only under the caster's direct control. To have an undead that is able to follow commands, you need to control a thing's mind rather than its body.
The thing is, as I see it, material objects have "inanimate spirits" you can affect magically. It's possible to stir the slumbering tree, to talk with a fire or stone, and so on. There is a distinction between affecting the mind and the body, however, just like a Rego Animal spell can cause the animal to move by affecting its body or by affecting its mind (with the same Art, Animal). So to have real undead, rather than animated puppets, you need to affect the body's inanimate spirit with a Rego Corpus spell, and grant it power to move the body (yet another Rego Corpus effect; compare with Free the Standing Giant and similar spells). If a stone or fire has personality traits and memories, I don't see why the body's inanimate spirit won't. So I assigned it some.
Unlike stones or fires, when a person dies his spirit becomes a Mentem entitry (of any Realm, but most often the Magic Realm), apparently. So it's certainly possible that a person's "inanimate spirit" doesn't exist (having been transformed into a Mentem spirit), or is unrelated to the person's past life (perhaps knowing only of it's death and what happened since).
So this opens up several kinds of undead:
- Direct control of the body through Rego Corpus. This is more like a puppet than an undead.
- Control and empowerment of the inanimate spirit of the body through Rego Corpus. The inanimate spirit might have the person's memories and senses, or it might not.
- Binding the person's Mentem spirit into the body and granting it the ability to move the body, a Rego Corpus spell with a Mentem requisite (using the body as an arcane connection to the spirit).
- Posessing the body by another spirit, by the appropriate spells. An example might be a Rego Vim spell to bind a demon to the human body, with perhaps a Corpus requisite to grant it power over the body. Another option is summoning (or creating?) a creature that can possess others, e.g summoning a demon and ordering it to possess the body.
- Making the body into a magical item, either allowing direct control via a Rego Corpus invested effect or binding a spirit into it via the Spirit Binding virtue (TMRE).
Note that Whispers Through the Black Gate allows you to speak with the body and spirit, an Intellego Corpus effect but with a Mentem requisite. (I'd actually peg it as a Rego Corpus/Mentem effect, as others can hear the spirit too.) So it doesn't help in resolving how much the body's inanimate spirit remembers and knows.
Well, as I see it, being an evil scumbag is partaking in the Infernal, the Infernal is evil and evil is Infernal. Otherwise, it kinda loses its point - you can have a person commiting all kinds of obscence vile things, selling souls to the devil, and so on - but no, that's got nothing to do with the Infernal Realm, he might very well be Divine... NOT. If anyone can do the evil stuff, then the Infernal suddenly isn't that evil, by comparison. It's just another form of magic. I much rather see it as the dark temptation, the taint of corruption. Anyone, of any Realm, may fall to evil - even god's greatest angel. And when you do, and to the extent you do, you become Infernal.
I agree creating undead shouldn't be evil per se. It's good to keep the options open for players. I do think, however, that creating them in a particularly god-defying manner (e.g. casting a spell over a Church's cemetary, or attempting to create the semblance of real life rather than undead servants) would be a sin of hubris. I'd make a check to see if it gives rise to an Infernal aura, and if it does I'll have the created undead subtly tainted by evil. Likewise for particularly vile acts, or repeated minor sins in the same locale (e.g., a magus pirate's ship might acquire an Infernal aura, with all his larceny...).