Ravana, Duryodhana, Kansa have two things in common. 1: They all are evil & 2: They are extremely powerful.
Why is evil so powerful when compared to a normal person? A normal person of Ayodhya or Hastinapur can never dream of getting rid of Ravana or Duryodhan.
I do not think your assumption is correct. Let me clarify why -
Ravana - he was not so powerful. kArtavIrya Arjuna manhandled Ravana very badly, he crushed Ravana in his powerful arms like how Garuda holds a serpent in his claws and there was nothing Ravana could do (You can find it in uttara kAnda of Valmiki Ramayana or read it here). vAli was way more powerful than Ravana, i.e. a vanara was more powerful than a descendant of Brahma. You can read that here. So how exactly is Ravana 'extremely powerfull'?
Duryodhana - he was defeated easily by Arjuna single handedly, you can read it in virata parva of Vyasa Mahabharata. He was also defeated by Bhima twice in Kurukshetra war per Vyasa Mahabharata much before the Drona was even killed. Read it here, which is a good summary of the war. He had to be constantly protected by Bhishma, Drona, Karna, Ashwadhama in the Kurukshetra war. Not 'extremely powerful' was he!
Kamsa - how did you get the impression that he was extremely powerful? He always relied and depended on hired help (per Vyasa Bhagavatam). At the end a boy ended up killing him. Not 'extremely powerful' is he!
Evil is as strong as good is, sometimes it is more stronger than the good and vice versa. And balance to equilibrium is reached quickly if that imbalance occurs, depending on which yuga it is. That is the norm. If that wasn't the case, then bhagawAn would not tell Arjuna in Vyasa MB BG that "..para dharmO bhyavaha ha" But evil is certainly not much more stronger than good is.
Prithvi loka is the realm where "Maya" tests us at every step by luring our senses. But the very character of our senses is that they are never satisfied (especially when senses in concoction with our own mind take control over our "Buddhi" ) this non satisfaction leads to anger (tamasic nature) which further destroys our budhhi and man submits to evil knowingly/unknowingly. Excessiveness of anger, action etc are linked to tamasic nature which increases the evil deeds manifold. (Lord Krishna called "krodha" to suck life out of "Pootna") Evil is never powerful that is why it is always defeated or keeps hidden.
Ravan: He used to be a "Parshad" of lord Vishnu and a door keeper of vaikuntha along with Kumbhkarna. he stopped sanathkumars to enter into vaikuntha and was given a shrap . later he was given a choice to be one with krishna after 6 births of being a bhakt or 3 births of being in opposition to krishna he chose the quicker option , (hiranyaksh+hiranykashipu) , (Dantavakra+shishupal). So no doubt he was powerful, but moment they chose evil they only became weaker.
Kamsa: though was a powerful asura only in his previous birth , he regarded vishnu as his enemy no.1. he was NOT all powerful though he had powerful asura friends and ministers.many yadavas had taken refuge in hastinapur when kansa was after them. kansa was only powerful to those who did not stand a chance against him and within his area.and due to jarasandha
Duryodhna: he was a slave of his senses, he was not an extraordinarily powerful but capable due to his bloodline, dronacharya's training and also due to lord Balrams training , he was insecure , he had means at his disposal but lacked strength .Bhim once threw an elephant into sky which never fell back on earth. evil is excessive but never strong enough As for a normal person to fight , perhaps rules of engagement differed.else eklavya was as good as arjuna , and sugriv's brother bali did defeat raavana.
