How much does Richard trust the knowledge of the novel he carried from his previous life? By now, absolutely zero.
He knew that everything written in the novel was real and happened at some point, however, there are things which happened differently than what was described in that novel.
There was a chance that one of the apostles had written and published it after reincarnating on Earth? To glorify the Protagonist, the being to whom they are devoted, they showed that Ethan sacrificed himself after using the Scythe, which belonged to Zulcriz.
There was a high chance.
That's why he couldn't trust the knowledge he carries. He has to ascertain through which means the God of Death was sealed the previous time so it doesn't happen again.
Regardless of whatever people believe, Richard knows that Zulcriz's existence is necessary. And not only because Richard was the inheritor of darkness but also because he knew the importance of Death.
