Why differentiate between intoxication and mental illness? Surely someone with severe enough mental illness that they're on the brink of suicide is much more suggestible than someone who is drunk.
IIRC, some articles frame this as a romantic relationship. That sort of involvement tends to give more than normal sway.
Desiring someone else's esteem or romantic affection should not be considered sufficient to make you their mindless thrall in the eyes of the law. The defendant was a juvenile peer with no legal, physical, familial, or economic control over the victim. She was just a horrible person he found attractive and persuasive.
There is a lot of space between mindless thrall in the eyes of the law and wholly irrelevant to the case.