Lenny with his memory intact had fore seen one major problem. Once the devil he was facing was destroyed, then the laws that governed this world would be gone too. This meant that he would lose his memories again and his soul would be in a mess.
Lenny had leant a lot, and even about the soul, coupled with his knowledge about Chaos magic and cores after consuming a lot of them, he figured that once he consumed the core of a Greater Devil, he was sure to advance in power.
But with the purple flame poison, this would be a headache, and if done wrongly, he could even end up injuring himself instead of increasing his power and abilities.
As such, Lenny was even willing to divide a part of the power to seal his soul. After all, he was still going to be facing against those other Devils.