"The Nirvana Sutra in essence teaches us that when persons who have faith in the Buddhist teachings and are determined to free themselves from the sufferings of birth and death grow a little faint in heart, the Buddha inflicts illness upon them in order to encourage them to strive harder. He does so to embolden them, to drive them forward."
In essence, this means that the Buddha inflicts illness to awaken a seeking spirit for the Law. Shin Yatomi mistakenly awakened a seeking spirit for Sensei rather than for the Law. He embraced not the Lotus Sutra and the teachings of Nichiren but rather the teachings of Ikedaism. Nichiren also teaches, "To give an analogy, a mild illness can be cured with ordinary medicine, but a severe illness requires an elixir". Late stage lung cancer requires the elixir of the Lotus Sutra not Ikedaism.
Likewise, he was an enemy of Nichiren. By altering and teaching an inferior religion [Ikedaism], he was ostensibly attacking Nichiren:
"Those persons who treat Nichiren as an enemy all attack him. The heavenly gods all [abandon] this country. [The Lotus Sutra] states, “[If this person] on seeing those who read, recite, copy, and uphold this sutra, should despise, hate, envy, or bear grudges against them,...” It also says, “[He will be] plagued by many ailments, thin and gaunt.” And the eighth volume states, “[And he will] suffer from severe and malignant illnesses.” The second volume also states, “Though he might practice the art of medicine and by its methods cure someone’s disease, the person would grow sicker from some other malady and perhaps in the end would die.” And it goes on to say, “If he himself had an illness, no one would aid or nurse him, and though he took good medicine, it would only make his condition worse.”
Lastly and most importantly, as taught by Nichiren, the most severe illnesses ["malignant illnesses"] are caused by slandering the Law and the votaries of the Lotus Sutra. Shin Yatomi slandered the Law and the votaries by claiming that not one member of the orthodox sects will attain Buddhahood through their faith and practice and that their copies of Nichiren inscribed Gohonzons have no power.