We must not accept any honour from oppressors
The most common way of helping the oppressors is to accept some post or position from them, especially if that post is a cause for any type of oppression. For example the post of maintaining law and order. If a person assumes such an assignment from the oppressors and as a result unleashes oppression on innocent people, such an appointment and such an acceptance of the post is a Greater Sin. If as a result of it, innocent people are subjected to hardships, the doer of such actions is sure to have a severe punishment from the Almighty.
There is one very well known tradition of Imam Ja’far as-Sadiq (a.s.) and it is quoted here from Tohful Uqūl. Imam Sadiq (a.s.) says: “The Harām post is the post of an oppressive ruler and the posts of those who are busy in the assignments entrusted by the tyrants. Hence it is Harām to fulfill the duty of this post. One who takes up this post will become involved in Divine punishment. Whether the job is significant or paltry, any attempt to help the oppressor is a Greater Sin. Because accepting a post from an oppressor will result in the trampling of rights, the manifestation of injustice, the spread of corruption and disturbance, the destruction of heavenly books, the murder of Prophets, the demolishing of mosques and the interpolation of religious laws. Therefore it is Harām to work with them. Except when there is a condition of utter helplessness, like when it becomes permissible to drink blood and eat the flesh of dead.” (Tohful Uqūl)..
Imam Ja’far as-Sadiq (a.s.) says: “If a person has authority in a particular affair of the people and he deals with justice with them and the doors of his house are open for those who refer to him and seek his help and he does not remain aloof from the people. Then the Almighty Allah will keep him safe from dread and fear on the Day of Qiyāma and make him enter Paradise.” (Wasa’il ul-Shia)
It must be remembered, however, that this is a vast subject and it is not possible to discuss all its facets. Hence those who wish to study in detail may refer to books of jurisprudence.