They ask you about the sacred month – about fighting therein. Say, "Fighting therein is great [sin], but averting [people] from the way of Allah and disbelief in Him and [preventing access to] al-Masjid al-Haram and the expulsion of its people therefrom are greater [evil] in the sight of Allah . And fitnah is greater than killing." And they will continue to fight you until they turn you back from your religion if they are able. And whoever of you reverts from his religion [to disbelief] and dies while he is a disbeliever – for those, their deeds have become worthless in this world and the Hereafter, and those are the companions of the Fire, they will abide therein eternally.
Ibn Kathiir has quoted Abu al-'aaliyah, Mujaahid, Sa'iid bin Jubair, ikramah, al-hasan, Qataadah, al-Dahhaak, and al-Rabii' bin Anas as saying that this means:
Shirk is worse than killing
Ibn Kathiir also says:
They used to torture the Muslims in regards to thier Diin to the extent that they reverted to kufr after having believed. And that is worse in front of Allaah than killing.
Sheikh al-Islaam Ibn Taymiyyah said: Killing consists of evil and wickedness but the fitnah of the kuffaar consists of evil and wickedness which is far worse.
So, the one who does not prevent the Muslims from establishing the Diin of Allaah, then the harm of his kufr is limited to himself.
And it is for this reason that the Jurists have said: The one who calls to bid'ah which opposes the Book and the Sunnah is to be punished with that with which the one who remains silent is not punished (because the one who remains silent, his bid'ah is limited to himself).
The majority have said that the meaning of fitnah here is the torture and torment the kaafirs subjected the Muslims to and pushed them away from their Diin until they were destroyed. This is far worse a crime than fighting them in the sacred month.