The IDF is the sovereign in the Occupied Territories, and bears overall responsibility for maintaining law and order there. It has the duty and authority to prevent violent and illegal acts of Israeli civilians against Palestinians and to arrest offenders.
The IDF has grossly failed to enforce the law against settlers and to protect the life, person, and property of the Palestinian residents from repeated attacks by the Israeli residents. The IDF's attitude towards cases of such violence ranges from deliberate non-intervention to more active cooperation.
IDF soldiers present during acts of settler violence against Palestinians do not attempt to stop or prevent these acts, or even to record identifying details of those involved and pass them on to the Israel Police. Soldiers' attempts to prevent crimes or arrest the perpetrators are rare and superficial. In some cases, the soldiers not only do not stop the violence, they actively take part in it.
In many cases, the IDF restricts the movement of Palestinians, including imposition of curfews, during which settlers attack the Palestinians. The IDF contends that these acts are taken to protect them, but they actually create an absurd situation in which restrictions are placed on the persons attacked ratter than on the attacker. Furthermore, the settlers take advantage of the fact that the Palestinians are under curfew and imprisoned in their homes to attack them and their property, which they are unable to defend. Thus, the IDF not only fails in its duty to protect the Palestinians, it also aids the settlers in attacking them.
http://www.btselem.org/settler_violence/idf