I would argue that they understand what's commonly accepted as being right and wrong, but they don't agree with it. If they actually thought it was wrong they wouldn't do it.
It does not matter if they agree or not about what's commonly accepted as being right or wrong. Human made choices, everyday we make decision. Every people who are addicted knew it's wrong choice to do, but they could not make the right decision due to many reasons. On the other hand, there are people who could make the right decision to stop their addiction and stood by it no matter what. Self determination to do what's right and the unwavering intention to do so have more role in people's tough moral choices.
He knew what he was doing, but he didn't have the full understanding because he didn't see the value in human life. If he understood that, he wouldn't have done it.
He thought he knew what he was doing, basically. But not.
How do you know that ? He could have known the value of human life, he could have understood all that, but he still did it anyway. Why ? He made the choice to do so, what brings him into making such decision is the real cause. This also means he knew exactly what he was doing, every choices that we make, we are fully aware of it, our mind, our consciousness made the decision, Holmes probably
decided to ignore what's right or wrong, ignore his own morality and all things that society deemed as right.
If he respected people's rights he wouldn't have violated them.
I am sure he respected people's rights when he wants to and chooses to, in that moment, he just chose to violate them anyway. Just like when soldiers in war choose to kill unarmed enemies who have surrendered, or when they rape women and kill children. A less extreme example : when someone "choose" to bullies the weak for fun and thrill, but came home and "choose" to act like a good kid who respected parents and all.
By the way the only proof required for my argument is that free will doesn't exist. If you would like to argue that it does, feel free. The simple fact that the brain operates and grows according to the laws of physics is proof enough that there isn't.
It doesn't matter what specific processes are involved in brain development. It doesn't matter that most people learn to respect human rights as they get older. This person didn't. Why?
Free will does exist. Human have the freedom to do according to their own choices, no matter how limited that freedom is.
Simple example :
People can choose to kill themselves, it's called suicide, sometimes they even choose to bring others together to their deaths in their attempt for suicide. Did they have free will ? They sure do, that's why they can make the choice to do so. They can also choose no instead of yes. Suicide bombers have the same brain like most of us, but they have been indoctrinated that their actions are justified, and they choose to follow their leaders.
I am sure not everyone who have been indoctrinated could come up with the same decision, some would just chickening out, some would choose no on moral grounds, some hold their principle so high that even death would not change their mind to do such atrocities.
Unfortunately, some would do the act of killing themselves and others like a bee circling a honey pot, they just put their own being higher than any other and choose to ignore everything else.
In every aspect of life, in every thing we do every day, when we give up to do what's right, when we give up in our struggle, every time we give up something or anything, we choose to do so, and not because of nature or fate.
To say yes or no is a choice, when we said that fate or nature caused us to give up doing what's right or give in to evil, we were just making sad excuses for the bad choices we made.