Humans rationalize everything. It’s how we exist. You cheat because “my partner isn’t giving me what I need.” If we allow for rationalizations to excuse this in any manner, then we will start sliding to lawlessness. For society to function properly, we need to agree on the acts that are wrong (murder), what might possibly excuse them (self defense), and what the punishment is (law & order sound 25 to life). We cannot give every person a free license to do what they want simply because their brains short circuit their own higher reasoning.
In addition, you are leaving out the fact that sociopaths exist. There are some estimates that up to 1 in 10 can be a sociopath. While this may be too high an estimate, it’s certainly possible for most people to enter a dissociative state at certain times, due to emotional state, illness, or injury. Anger is a common one. Ever been angry enough at someone that you hope or wish they die? Ever been so enraged at someone that if they died in a fire at that moment, you’d feel no empathy for their suffering? How about depressed and wish the world to just end?
A lot of people have. Should we let you off the hook because you are enraged and the object of your rage is on fire in front of you while you clutch an extinguisher? I don’t believe so. I feel confident in opining that most people would at least express trepidation with letting you off the hook.
As for “knowing” it’s wrong, that’s not really the issue. Even if you have no moral compass whatsoever, you know what the law disallows. If it’s direct violence or direct deprivation of another, it’s pretty much illegal. I’m sure we could hunt up loop holes and such, but it’s not like you can go into court and go “Your honor, I didn’t know what beating my spouse with a lead pipe was illegal.” which is why everyone goes “I was blinded by rage at finding her cigarettes on the floor! Please grant me a reprieve!”
Despite possible leniency in certain circumstances, direct responsibility must be the root of justice. If you cause any kind of harm (physical, financial, etc) to another, you must be punished.
(I further believe that this must also be accompanied by equality in dispensing that justice, but that’s slightly off topic.)