What is it about the murder of macduff's family that causes many of the thanes to leave Macbeth?

Probably its pointlessness. Macbeth accomplished nothing by killing Macduff's family. They were no threat to him, and killing them only made Macduff more angry and determined to have revenge. It also involves the killing of children. Then, as now, crimes against children are considered among the worst. The play is full of images of the murder of children: the finger of the birth-strangled babe, the baby whose brains would be dashed out by Lady Macbeth, Fleance, on whom a murder attempt is made, and Macduff's son who bravely puts up a fight against the "shag-eared villain". The Macbeths represent an attack on the future generation by the present, as unnatural child-killers. Macduff, on the other hand is a son who has killed his mother (mothers invariably died after a ceasarian birth at that time).