In fairness, I think that we need to say that there is a difference between a mental illness where the person has no control over his/her actions, and someone who simply does not care about right & wrong.
I only make the point because, many–not all–serial killers can stop killing any time they want to, & often do, for longer or shorter periods of time.
If we are going to talk about accountability, about the “mental means of committing a sin”, I think we also have to take note of the fact that just because someone has a mental illness does not automatically make that person unable to understand that what they are doing is wrong, nor does it take away their free will.
I think that this is a very complicated area, & it is important not to generalize too much.
Some criminals may not have any understanding of what they are doing, like a child.
Others may well just want to do horrible things, & only care about whether they get caught–2 very different kinds of people, though neither comes under the heading of normal.