(In their hearts is a disease; and Allah has increased their disease: And grievous is the penalty they (incur), because they are false (to themselves).)
According to Tadabbur e Quran, surah al baqarah pg. 135:
The word marad (sickness) is generally used in the Quran in two senses: (1) rancour and envy, and (2) hypocrisy. Where it is used along with the word nifaq (hypocrisy) it always signifies rancour and envy. Where it is used on its own, it either covers both the senses, or is used in either of the two senses as defined by its context. Considering its present context, the word here clearly signifies envy and malice.
The act of increasing their jealousy here is attributed by Allah to Himself. However, it refers to the Divine law according to which a person who insists on nursing malice in his heart, rather than purifying it to receive the light of faith and guidance, will find himself in circumstances that further add to his venom, envy and rancour.
Islahi, Amin Ahsan. Tadabbur e Quran. (p. 135 surah 2 verse 10). Surahs-1-2-and-Surahs-31-50.pdf (islamicweblibrary.com)