Zio did not kill Alys

OAKLEY : I understand what you're saying ! You're right
and I love them both too(thay are both meeting their fate in the game...)
BRAGATYR : Right too ! Zio's music boss is one of the best ever created for me too.

Zio seems to be merely a human-like manifestation of Dark Force, though. At least, that's how I always interpreted him.

MMXPERT wrote:

it is unknown if Zio is to blame for it, dark force was controlling him to his death, unlike say...Rulakir who came to his senses before passing away. either Zio was a true worshiper or the corruption was too strong, once again, we cannot know.

The concept art for Zio shows that he was physically corrupted as well- underneath his armor, parts of his body have been transfigured to resemble his master more than anything human.

There's also that true worshiper/mental corruption are not mutually exclusive, as that's basically how you get Cthulu cultists: He encountered something that Man Was Not Meant To Know and then embraced the gospel of oblivion as the only sane reaction to an insane universe. With a little nudging from the big D on where to apply his apocalyptic fervor, of course.