I am a doctor working in a government hospital with other Muslim and Non Muslim doctors of different nationalities since last one and a half years.
My unit head is a Muslim who till now was very nice with me in fact he was closest to me but he has a very bad habit of unnecessarily harassing other Muslim colleagues who do not accept his ideas. He takes undue advantage of his position as the unit head to harass them and plays dirty politics. But Alhamdulillah I was the only one who always used to tell him frankly that what he is doing is not nice and as a Muslim he should not do it.
In spite of me very frank in condemning him it looked as if he had good relations with me, may be he was afraid of my angry young man status.Yes I have a very bad habit of going out of control some times. Alhamdulillah when I do anything I try my best that it is to make Allah happy and for this many a times I tolerate to the maximum anything which is against me. For example when some Muslim brother hurts me or deceives me, in spite of having a right to take revenge I forgive him thinking that the person in question is my Muslim brother and for ittihad among us I need to sacrifice something. But sometimes after tolerating a lot when it becomes unbearable I suddenly burst and cannot control myself.
As usual, about two weeks back i had a discussion with this man in which I criticized him for his dirty politics and he accepted his faults and we departed As good Muslim brothers but since then his tone has changed and suddenly Unexpectedly he has turned his guns on me and is making friends with his rivals against whom he used to play dirty politics. Alhamdulillah I have good relations with all my other colleagues but now this man has started his dirty politics against me. Alhamdulillah I am tolerating everything at present and I am not doing any negative campaigning against him which he may be doing right now. In his previous campaigns against the most descent and religious brother among us he had made him famous among the hospital staff as a MUNAFIQ and in fact used to call him by this name in front of others but in spite of his negative campaigning against him he could not harm him and he is the most respected colleague. But now to my surprise he has made the very man he used to call a MUNAFIQ, his best Friend.
Now brother can u guide me in the light of Quran and Sunnah that what shall I do? Shall I continue to tolerate and leave it to Allah? Which my conscience tells me right now or shall I teach him a lesson, which a part of my brain advises me to do? Please guide me in the light of Quran and Sunnah.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

May Allah reward you for your enthusiasm regarding keeping good relationships and refraining from bad conducts and immorality. May Allah increase your zeal in doing so.

You have to know that forgiving your boss, and performing a good deed in return for a bad deed, as well as reconciling, are conducts that you should keep high. Allah is able to make the relationship between both of you better even after an instant. Allah says:"Let them pardon and forgive. Do you not love that Allah should forgive you? And Allah is Oft-Forgiving, Most Merciful. "(An-Nur 24:22)

 And you have to know that Allah promised a great reward for the person who forgives the mistakes and shortfalls of his brothers, Allah says:" And let not those among you who are blessed with graces and wealth swear not to give (any sort of help) to their kinsmen, Al-Masâkîn (the poor), and those who left their homes for Allah's Cause. Let them pardon and forgive. Do you not love that Allah should forgive you? And Allah is Oft-Forgiving, Most Merciful. (An-Nur 24:22)

Also Allah says:" The recompense for an evil is an evil like thereof, but whoever forgives and makes reconciliation, his reward is due from Allah. Verily, He likes not the Zâlimûn (oppressors, polytheists, and wrong-doers, etc.)". (Ash-Shura 42:40)

Allah further says:" And verily, whosoever shows patience and forgives that would truly be from the things recommended by Allah." (Ash-Shura 42:43)

Allah also stated the characteristics of the people saying:" Those who spend [in Allah's Cause - deeds of charity, alms, etc.] in prosperity and in adversity, who repress anger, and who pardon men; verily, Allah loves Al-Muhsinûn (the good­doers)." (Aali Imran 3:134)

The prophet (sallalahu alaili wa sallam) also said regarding forgiving: "Sadaqah(Zakah and alms) never decreases wealth, and the more a persons pardons people, the more Allah raises him in status, and the person who is humble for the Sake of Allah, Allah will raise him in status,"  reported by Muslim.

You have to advise your boss with the juristic etiquettes of advising, like choosing the best words, as the good words are sadaqah. You should not advise him in public, as it is better to do so secretly, because giving advice in public is humiliating to the person. Try to choose the best ways and means in calling him (to Islam), you can, for instance, give him some tapes and booklets which talk about backbiting Muslims and being unjust to them, and similar things which you noticed him doing. You can also agree with an Imam to talk about this subject in the following Friday Khutba, and so on.

But do not become angry. Abu Huraira narrated "that a man came to the prophet and said to him "Advice me "The prophet told him "Do not become angry;" the man repeated the same question. The prophet replied with the same thing several times: "Do not become angry." reported by Al-Bukhari.

As you know, when a person becomes angry, he loses control of his/her self. He might act in a way that he would later regret.

We ask Allah to enable us and you to do whatever He loves and He is pleased with.

Allah knows best.