The Quran says the hearts of men are darkened with their sins.
No ‘good’ or ‘bad’ deed is ever left unreturned!
Even an ill ‘thought’ has consequences!
“Whether you show what is within your consciousness (your thoughts) or conceal it, Allah will bring you to account for it with the quality of the Name Hasib.”
Verses revealed after the above verse do not invalidate this warning; however, they state one is not responsible for what one can’t control. That is, you’re not responsible for a thought that may randomly pop into your head, but once you take it seriously, energize it, identify with it, and follow it through, that’s when you activate the mechanism of consequences!
The interesting thing is, most people do not even realize the adverse situations they face are the direct results of their own doing! If this was truly known, then faith would become manifest by default. The system is designed so this mechanism of consequence isn’t easily observable.
The great majority of people live with an imitative approach to religion, putting all their effort into the transitory things they will leave behind.
The system of subjecting one to the consequences of their deeds while still on Earth is called the ‘divine scheme’ (makr) in the Quran.
Depending on the person’s intention, the consequent energy will be returned to him for as long as forty days to forty years, and in some cases, until the end of his life. This is called the darkening of hearts. If this action leads him to leave this abode without faith, then his heart has darkened (his discernment has been blocked), which generally refers to an inadequacy of insight to evaluate the truth.
Notice that the prohibition is in reference to an ill thought, a negative thought. That is, harboring thoughts and opinions about others that they don’t deserve. Thoughts are also actions; they are the actions of the brain. And each person is responsible for their actions, i.e. their consequences are inevitable.
When a ‘negative thought’ first comes into one’s mind, the individual is not initially responsible for it, but if they continue thinking the same thoughts, then the system necessarily begins to lock and block the brain.
The results of unduly blaming someone can range between denying the judgment and will of Allah, to losing faith altogether. And if the person dies in this state, he will make the transition to the next world as an unbeliever.
The biggest symptom of a darkened or blocked heart is an imitative approach to things, based on memorization rather than authentication, and the pursuit of material pleasures and gain – generally ranging between the bedroom, kitchen and workplace. Such people squander their lives with things that will reap them no benefit at all in the afterlife. In fact, they may even be more arrogant than before, which is the result of the divine scheme!
Sadly, however, the person is unable to fathom this, he is unable to see that he has been cursed by Allah (dispelled away); on the contrary, he thinks he has a pure heart and has thus been subject to the blessing of Allah! When he’s told, he fails to understand; when grace is showered upon him, it dries away before he is able to receive it and benefit from it.
He is unable to comprehend how far he is from knowing the One whose name is Allah and how far he is from living his daily life through His viewpoint, how much suffering he is subject to because of his inability to discern the reality of fate and faith, and how all of this will lead him to hellfire in the afterlife.
If one says something offensive to the Rasul of Allah (saw), even if inadvertently, he will become blinded to the truth. If one says something offensive to any of the saints of Allah, whether consciously or unconsciously, he will be deprived of the light of sainthood altogether. Not because some exterior power will punish him, but as the automatic output of the system requisites.
A wrong thought is the direct result of a state that is contrary to the principles of faith. Continuing to dwell on thoughts that contradict the principles of faith darkens the heart even more, preventing the person from experiencing the reality. This is self-afflicted punishment!
 Quran 2:284