Spoilers about Undertale’s genocide run:
Sans is the final boss. He's convinced if you get past him you're going to kill every living thing in the world, and he reasons that would be bad. And he's clinically, profoundly depressed, so he doesn't care about anything. Really, actually doesn't care.
So he has no problem fighting you as many times as it takes for you to give up. He makes it as boring as possible for you, not saying much - he starts out telling you how many times you've reloaded your save, just so you know he's keeping count, and you understand it doesn't bother him.
And he has no problem cheating like Hell. He fights you with every unfair, unfun trick imaginable: he dodges over 95% of your attacks, which the game apparently allows, it's just that no one else does it. He ignores your i-frames and damages you 30 times per second. With stacking poison damage no one else has. The variety and speed and length and size of his attacks are on a whole different scale from everything else in the game. Reportedly it takes an average of 1-2 weeks of practice for people to beat him.
Me, I just gave up. I don't believe I physically have the ability to do the fight. But also it is of course the better choice. Sans weaponizes his depression in a relentless, never ending, soul crushing, anti-fun boss fight designed to wear down the most persistent player to save the world from you, and I'm happy to report he beat me.