When you say nothingness most people think that means just nothing. What it really means is nothing happens. What you did in life didn't matter, you were dead. Deal with it. So when Mello woke up he was expecting hellfire and brimstone, not a nice bedroom with a red head sitting next to him playing a DS. "Hey." Matt muttered, smirking at Mello's dumbfounded face. Well, death just became interesting.
When they met Near they were surprised to learn he'd died a year after them, time flew when you had everything you'd wanted and more. Mello had learned everything was free and now constantly raided the candy store, Matt was an official game designer. As in, it was HIM who made the games of Mu... of course!
Perhaps that was also the punishment though, because when you want for nothing you become bored. It was quickly learned you could do anything and nothing, some children still attended school because they didn't want to accept they'd died. Adults became teachers just to relive their lives. There was no crime; Mello wondered why he wasn't happier. It was years before he realised.
He realised too late that you could die in Mu, but only when it let you. It was how things worked, the person was reborn back in the human world and when they died they'd return to Mu... without their memories. He watched Matt die and come back not knowing who Mello was.
Perhaps Mu was invented to punish you for being human.
Perhaps Mu existed just because it did.
Or perhaps Mu is only a dream invented in your mind to escape reality.
You live. You die. And you're alone all that time. Mello watched Matt in the distance laughing and hoped Mu would let him die soon, so he could forget the memories of his love who never returned his feelings. Maybe that was Mello's punishment, or maybe it was Matt's gift.
You live.
You die.
And that's all there is to life.