And when it is said to them, "Do not cause corruption on the earth," they say, "We are but reformers."
[Qur'aan 2:11]
And when it is said to them, "Do not cause corruption on the earth,"
This means that they were told not to act disobediently in the earth. Their corruption was their disobeying Allah. This is because the one who disobeys Allah or orders something which is related to the disobedience of Allah, corrupts the Earth. The preservation and well fare of the earth and Heavens is in the obedience of Allah.
The hypocrites would disobey Allah in the earth, leading to its corruption. They would commit acts which Allah forbade them from. They would forsake the obligations which Allah had ordered mankind to full fill. They would try to create doubts regarding the Religion of Allah, a religion which one must accept if he/she wants his/her deeds to be accepted. They would try to infuse into the hearts of the believers doubt regarding their Religion.
Hasan said: From among the things which lead to corruption in the land is when the believers take the disbelievers as 'awliyaa.
Allah has cut of any sort of muwaalah (friendship) between the believers and the disbelievers.
The danger of the hypocrite is great. He causes the greatest corruption in the land because on the outside he appears to be Muslim while on the inside he is friends with the disbelievers. If he remained on his original state, both in appearance and on in the inside, the danger he poses would be less.
Reference(s):
Ibn Katheer