I came to the conclusion to end the campaign prematurely. It wasn't an easy decision and was the result of my short temper, as well as conflicts within the group and a generally disrespectful behaviour towards me as the GM. I may post a topic explaining this later on and ask the forum how to avoid it or something similar. (Stay tuned and find out!)
Anyway, the cyberzombie was supposed to be "processed" from a former friend of the players, one they had witnessed go down like a hero as he saved them. He'd have been tortured and brainwashed almost beyond recognition, but once the players discovered his original identity killing him would also feel horrible, despite him being their nemesis.
<Mass Effect 3 spoilers>
Imagine Jack if not saved from Grissom Academy, converted and turned into a Phantom. That's the tone I wanted to create.
<End of spoilers>
Anywho, feel free to use the concept in a campaign and please inform me about how it worked. Also, does anyone know how to format text for Spoiler alert, so that you have to click in order to see it?