Is it necessary to grieve for a brother who has destroyed himself with acts of disobedience?

One must strive to guide the disobedient brother and lead him to repentance and rectitude. He should not be shunned unless there is fear for one's religion, or if shunning would lead to his repentance. If shunning has no effect on him, then there is no benefit in it; rather, it contravenes the methodology of the prophets.

Indeed, shunning those who commit sins is not an end in itself; rather, the objective is the rectification of people's affairs. If shunning leads to the rectification of the sinner's condition, then it is good or obligatory. But if interaction leads to those same benefits, then interaction is preferable.

As for grieving over sinners, it is a natural feeling that arises from compassion and fear for them concerning punishment. This happened to the Prophet, peace be upon him, whereupon Allah revealed to him: "And do not grieve over them."