Its never proved that Mello used the notebook. He did. Only once.
Mello's life was full of sins. He killed. He lied. He slept with people he barely knew to gain the title of a Mafia boss. He was surely going to hell. No one had ever been there to tell him that everyone ended in MU. But Mello had survived so much pain, what could possibly hurt after that?
Matt. If he lost Matt, if he never saw Matt again, it would hurt. It would be he's own, personal hell.
When Mello still had the notebook, he ripped out a page. It survived the explosion.
Mello got a phone call from Lidner, telling him what Near planned to do. Mello found a hole in the plan, saw the mistake Near would do that would kill everybody in the room. Except from Kira.
Mello also saw what he had to do to fix the mistake. What he would have to sacrifice.
No one wants to die alone. Humans are scared of the unknown, and death sure as hell is one of those places! You just don't want to face death alone...
Mello knew Matt would survive. Takada's bodyguards wouldn't be able to catch him. He's car was fast, and they would be left in the confusion of his smokebomb.
Mello didn't want to face death alone...
Its never proved that Mello used the notebook. But he did. Only once.