1)I've made my character under the assumption that Life + Dark = Mutation, more or less. Am I correct on this?
There are many ways to combine multiple elements, so yes, Life + Dark can be mutations. Another common combination of the two is undeath.
2)Expanding on that, how would trying to offensively mutate others work? Like an infection which could transform/mutate the victim into a feral abomination. That is sort of a mind control, even though that would never specifically target the mind - it's the body and the brain that were forcibly changed.
This... wouldn't work very well, to be honest. Mutations tend to be extremely unpredictable, so you're almost never going to know what kind of effect you're going to inflict on your target. Sure, the mutation could damage the victim's body, or drive the victim insane, but it could also make the victim stronger; it's generally too unpredictable to be very useful in a fight against someone on a similar power level as you. It might be useful against large swarms of relatively weak enemies, where you turn one of the mooks into an insane rampaging monster that attacks the rest of the mooks, but that's a bit too situational. Against normal enemies, you're better off sticking to poison or something else that's guaranteed to inflict damage to the target.
Also, brain damage can indeed damage the target's mind. But since the mind is not being directly tampered with, you can't really control
how the mind is going to get damaged. The victim will most likely be driven insane, yes, but the insanity could potentially manifest as an insanely powerful obsession to exact vengeance against you at all costs, or some other kind of thing that actually makes the victim more dangerous. As I said, it's far less reliable.
3)What's an ambitious bioengineer with no morals without a zombie virus. How realistic would it be? All these assumes John studies/analyzes the species he wants to infect beforehand. Maybe even tailors his mutagens/viruses for specific races?
At least one magical zombie virus already exists in the Cradle, which kills its victim and reanimates the corpse, so it's possible for your character to learn that as an ability and develop his own variants. Magical pathogens like this can potentially infect any being that has mana vessels, so you won't need to tailor it for specific races. But some beings (e.g. inorganic beings in general) are inherently more resistant to Life-based magical pathogens, and you can't really do anything about that aside from making your pathogens more efficient and/or powerful.
@gammaflux: Your character is accepted.