Someone, who kills those non-Muslims who are living in Muslim society, will not even get the scent of heaven. Attacking place of worships for [G1] not Muslims[G2]  including monasteries, churches, synagogues is Haram and impermissible in the eye of Islam. It is a harshly punishable offense.  [G3] [G4] [G5] 

( وَلَوْلَا دَفْعُ اللَّهِ النَّاسَ بَعْضَهُم بِبَعْضٍ لَّهُدِّمَتْ صَوَامِعُ وَبِيَعٌ وَصَلَوَاتٌ وَمَسَاجِدُ يُذْكَرُ فِيهَا اسْمُ اللَّهِ كَثِيرًاوَلَيَنصُرَنَّاللَّهُمَنيَنصُرُهُإِنَّ اللَّهَ لَقَوِيٌّ عَزِيزٌ ) (الحج: 40)

For had it not been that Allah checks one set of people by means of another, monasteries, churches, synagogues, and mosques, wherein the Name of Allah is mentioned much would surely have been pulled down. Verily, Allah will help those who help His (Cause). Truly, Allah is All-Strong, All-Mighty. {Sura Hajj:40}

في تفسير ابن عباس (صوامع) صوامع الرهبان (والبيع) كنائس اليهود (وصلوات) بيت نار المجوس لأن هؤلاء في مأمن المسلمين.(تفسير ابن عباس). و قال القرطبي في تفسير هذه الآية: تضمنت هذه الآية المنع من هدم كنائس أهل الذمة وبيعهم وبيوت نيرانهم. (أحكام القرآن). و قال الماوردي في تفسيرها: لهدمها الآن المشركون لولا دفع الله بالمؤمنين.

It is reported in Tafsir e Ibne Abbas that, Sawame meaning place of worship for Christian priests, Bia’a meaning for the Jew, Salawa meaning for pagans. Such establishments can never be destroyed because their security is entrusted upon Muslims.

Qurtubi, while explaining this ayah, stated, this ayah proves that it is prohibited to destroy a place of worship for Christian priests, Jews, and Pagans. [G6] [G7] 

Mawardi stated in his Tafseer, had not Allah protected these places through Muslims, polytheists would have destroyed them by now.

“فقد صرح في شرح السير بأنه لو ظهر في أرضهم وجعلهم ذمة لا يمنعون من إحداث كنيسة”. (رد المحتار).

It is mentioned in Sarhussiyar that, if Muslims won over not Muslims[G8]  and they enter into Zimmi agreement, then Muslims cannot forbid them from building their place of worships. {Raddul Mukhtar: 6/328}[G9] 

ومن مقتضيات عقد الذمة أن أهل الذمة لا يظلمون ولا يؤذون. قال النبي: “ألا من ظلم معاهداً أو انتقض حقه أو كلفه فوق طاقته أو أخذ منه شيئاً بغير طيب نفس منه بأنا حجيجه يوم القيام”.

It is the essence of Zimmi agreement that, they cannot be oppressed or hurt.

Narrated by a number of Companions of the Prophet:

Safwan reported from a number of Companions of the Messenger of Allah (PBUH) on the authority of their fathers who were relatives of each other. The Messenger of Allah (PBUH) said: Beware, if anyone wrongs a contracting man, or diminishes his right, or forces him to work beyond his capacity, or takes from him anything without his consent, I shall plead for him on the Day of Judgment. Abu Dawud (1603).

الأصل أهل الذمة تركهم وما يدينونه فيقرون على الكفر وعقائدهم وأعمالهم التي يعتبرونها من أمور دينهم كضرب الناقوس خفيفاً بينهم ولا يمنعونهم من ارتكاب المعاصي التي يعتقدون بجوازها كشرب الخمر واتخاذ الخنازير وبيعها أو الأكل والشرب في نهار رمضان وغير ذلك فيما بينهم.

Every Zimmi non-Muslim citizen living in a Muslim society must be given freedom of religion. He can maintain and practice his belief and customs such as ringing the bell etc. Muslims cannot refrain them from such activities which are permissible in their religions such as drinking alcohol, trading swine etc. {Binayah: 4/807, Shami:3/272}