What does it mean that no believer should be killed in retribution for killing a disbeliever? Does this mean that someone could kill 100 disbelievers and suffer no repercussions?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

Scholars held different views regarding the killing of a Muslim in Qisaas (retaliation) for killing a disbeliever. We have previously clarified this in fatwa 92261. This is as far as the first part of the question is concerned.

As for the second part of the question, the answer is that even according to the scholarly view that there is no Qisaas for killing a disbeliever, this does not mean that Islam endorses such killing or that it is not considered a crime. On the contrary, Islam prohibits the killing of any protected human soul, even if it is the soul of a disbeliever. Rather, the prohibition is more strictly emphasized in the case of a Thimmi (non-Muslim living under protection of the Muslim state). ʻAbdullaah ibn ʻUmar reported that the Prophet, sallallaahu ʻalayhi wa sallam, said, “Whoever kills a Muʻaahad (a person who is granted the pledge of protection by the Muslims) shall not smell the fragrance of Paradise though its fragrance can be smelt at a distance of forty years (of traveling)…” [Al-Bukhari]

Islam prohibits inflicting harm on the Thimmi/Mu'aahad, let alone killing him. The Prophet, sallallaahu ʻalayhi wa sallam, said, “Beware, if anyone wrongs a Mu'aahad, diminishes his right, burdens him beyond his capacity, or takes from him anything without his consent, I shall plead for him (the Mu'aahad) on the Day of Judgment.” [Abu Daawood] Ibn Hajar Al-Haytami wrote, “As for the prohibition of harming a Thimmi, it is prohibited to inflict harm on the Thimmi just as it is prohibited to inflict harm on the Muslim because the Islamic sharee'ah protected his life, honor, and wealth.” [Az-Zawaajir ʻan Iqtiraaf Al-Kabaa'ir]

The Muslim who kills a Thimmi is liable for Kaffaarah (expiation) and Diyah (blood money), just as he is liable for them in case of killing a Muslim. The expiation in this case is freeing a Muslim slave, and if that is not possible, then he should fast two consecutive months. Allaah, The Exalted, says (what means): {...and if he was from a people with whom you have a treaty - then a compensation payment presented to his family and the freeing of a believing slave. And whoever does not find (one or cannot afford to buy one) - then (instead), a fast for two months consecutively, (seeking) acceptance of repentance from Allaah. And Allaah is ever Knowing and Wise.} [Quran 4:92] Ibn ʻAbbaas, Ash-Shaʻbi, An-Nakhaʻi, Ash-Shaafiʻi, Ibn Jareer At-Tabari and others held that a Muslim who kills a Thimmi is liable for expiation.

Allaah knows best.