The Midrash answers that Kayin had no one to learn from. Therefore his punishment wasn't so severe. But from Kayin onwards anyone who murders would be punished to be killed.
We all know that Hashem punished Sedom and wiped them out completely. They were cruel, anti-Chessed, anti-kindness, but to deserve to be wiped out completely? Were they worse than Kayin who was punished like someone who killed by mistake? Who were they meant to have learned from?
Maybe the following story will give us a little understanding.
A group of Jews arranged to travel to Rabbi Chaim Halberstam of Sanz to receive his blessing. They davened early in the morning, set out on their journey, planned to see the great Rabbi straight after he finished praying. When they arrived the Rebbe was in the middle of his prayers. The Rebbe davened with intense concentration, shook, clapped his hands while he was davening.
The guests stood at the back watching the Rebbe with awe as he prayed.
One elderly man, quite amused by the sight, made his way all the way up front to get a close up view of how the Rebbe was davening. He got closer and closer till he was all the way at the front next to the Rebbe and staring him in the face.
Suddenly, the Rebbe stopped, grabbed the elderly Jew, walked him to the back and threw him out the Shul.
The Rebbe returned to his place and continued praying like before.
After davening, the guests stood in line and the Rebbe blessed all of them. However, when he saw the old man, he grabbed him by the arm and threw him out again.
The Rabbi entered his room and outside the elderly man was crying bitterly.
The Gabai entered the Rabbi's room and told him that the old man is waiting outside.
The Rebbe replied, "he should go back home, I don't want to see him."
"But he is crying bitterly." Said the Gabai.
"That's what he should be doing." Replied the Rebbe.
"But he wants to know what he did wrong," said the Gabai.
"When a Jew is davening and serving his Creator, does one looks at him as if he is an interesting animal in the zoo? That it had no impression on him at all, just entertained and amused? Someone that after watching another fellow Jew praying and it has no impact on him at all, shows that he has no spark left in his soul at all. Such a person, I don't want to meet!"
"But he is crying," said the Gabai, "so it shows that his souls has been aroused."
"If so," said the Tzaddik, "he can come in for a blessing."
The people of Sedom were neighbors of Avraham Avinu, the pillar of Chessed, the example of true kindness. To be like Avraham Avinu, is impossible, but that it had no impression on them what so ever? That shows that any feeling for humanity didn't exist by them at all and there was no hope for them what so ever. They lost their right of existence.
