To kill the main character, he needed something that could make his life saving treasure useless. He needed something that could prevent the main character's escape when he was on the verge of death.
Even in his wildest dreams, he didn't want to give the main character an opportunity to escape as he knew it was a recipe for disaster.
In almost every fantasy novel he read, there was always a villain who was too strong for the Main Character. But they allowed him an opportunity to escape, only for the main character to return stronger and kill them.
This was the last thing Eren wanted, hence he had been delaying his plan. But if he could make sure that the Main Character wasn't able to escape, he could finally take the last step.
According to the Author, there was no Transmigrator in the Eastern Continent except him. Once the Main Character was gone, this was more or less a safe space for him, as long as he was cautious.
