Powerful MC's

King Grey has unrivaled strength, wealth, and prestige in a world governed by martial ability. However, solitude lingers closely behind those with great power. Beneath the glamorous exterior of a powerful king lurks the shell of man, devoid of purpose and will.

Reincarnated into a new world filled with magic and monsters, the king has a second chance to relive his life. Correcting the mistakes of his past will not be his only challenge.

Raul ho had defeated the demon king as the strongest hero should have become the savior of the world.

Having his treasured companions and family killed by power hungry aristocrats, he was executed with false charges brought against him by the princess. Just before his life collapsed, at last his heart fell into darkness.

"...I will tear those traitors from limb to limb. Burn them at the stake. Sever them to pieces. Skewer them. I will kill every one of those bastards without mercy.

Gamin Yoon, a 17 year old student, goes to a school known to harbor future criminals. Stuck in a carnivorous environment where only the strongest survive, he struggles to get good grades to enter a 4 year university. But, will he be able to study well enough to enter university, or will he get involved in the unending fights around the school?

Hunter Sung Jin-woo started out as a weak E-rank hunter barely able to provide for his sister and mother's hospital bills. But after a failed dungeon capture attempt, he's suddenly able to get more powerful thanks to this 'system'.

I dropped it on my first read actually. I think I was insane because this is literally SO GOOD.

The worst student of Westrod Academy, the disgrace of the Blake House.
A new soul enters the lifeless body of Frey Blake, the boy who couldn’t handle his painful life and chose death instead.
“What’s with this body?
I’ll have to check every little detail of it.”
The Great Mage Lucas Traumen.
He earned his chance at revenge by gaining the flesh and bones of Frey Blake the disgrace!
And now begins the interesting adventure of the Great Mage who returned after 4000 years!

A boy who is bullied, and forsaken, is chosen by a sword that falls from the sky.
He can now get revenge on the bullies who ruined his life.

When I opened my eyes, I was inside a novel.
[The Birth of a Hero].
[The Birth of a Hero] was a novel focused on the adventures of the main character, Choi Han.
Too bad I became the Trash of the Count family who ends up meeting his demise at the hands of Choi Han. No way is that gonna happen. Let's live a good life away from all the action.

You can probably guess how that turned out.

At a certain park, there is a mysterious man, who has red eyes and a black suit, that people can go to and request him to kill someone. His assassinations can never be blamed on him because he takes no illegal action. His strength is in the power of suggestion and the arrangement of events. He accepts these requests, all the while privately mocking the foolishness of those whose unwise wishes he grants.

The only thing that fits this list is the powerful MC part, but whatever.

The manga centers on homicide detective Hiroki Dan, the son of a former police superintendent general, and who seems to be on the fast track to career success. He maintains a hidden identity as a serial killer responsible for more than a hundred murders of people he deems beyond the reach of the law.

”Is it okay to kill people?"
"Shun, if someone is converted, let him go and do nothing. He should be given a chance to be reborn."
After a group of bullies ends up killing his entire family, Shun trains under his grandfather and swears to take revenge upon all his enemies.
"I won't let ANYONE get in the way of my revenge."