Rav Eliashiv: Moderation is normally required but extremism is required in times of spiritual danger BK (60b)

Bava Kama(60b): Our Rabbis taught: When there is an epidemic in a town, one should not walk in the middle of the road, as the Angel of Death walks then in the middle of the road, for since permission has been granted him, he stalks along openly. But when there is peace in the town, one should not walk at the sides of the road, for since [the Angel of Death] has no permission he slinks along in hiding.

This is explained by Rav Eliashiv (Yeshurun #28) in his commentary to Avos to refer to one's approach to religious observance. This is elaborated by Orech Yesharim.

Orech Yesharim (brought by Daf ahl Daf to Bava Kama 60b): As a general rule a person should always go in the moderate path and not be an extremist. However when there is an epidemic in the city i.e., there are many people who deviate from the proper path of observance – then one can no longer take a moderate position and he should not follow the majority in sin. Rather he should take an extreme position. That is because the Angel of Death who is Satan can easily seduce people to sin at this time. It is only during a time of peace i.e., when people are observant and attached to G‑d – then it is necessary to take a moderate position. In fact if a person takes an extreme position during times of general religous observance he places himself in great spiritual danger. Because at that time Satan has domain over those who conduct themselves as fanatics and extremists since their conduct is not for the sake of heaven.