A good many people of any faith don't bother to read their scriptures. If the Christian couple did, there's a chance that the baby would have lived; if the Muslims did, there would either be more murders, or more of them leaving the religion.
So what does the Quran mandate? 9:29 "Fight against such of those to whom the Scriptures were given as believe neither in God nor the Last Day, who do not forbid what God and His apostle have forbidden, and do not embrace the true Faith, until they pay tribute out of hand and are utterly subdued." 9:73 "Prophet, make war on the unbelievers and the hypocrites and deal rigorously with them." Just two of the verses against disbelievers. Translation: N.J Dawood. If you think the translation's wrong, I can give the Arabic; if you think I've taken them out of context, show me.
From Al-Bukhari, an important hadith (a record of Mohammed's views and actions) 9:17: "Narrated Abdullah: Allah's Messenger said, "The blood of a Muslim who confesses that none has the right to be worshipped but Allah and that I am His Messenger, cannot be shed except in three cases: in Qisas (equality in punishment) for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (Apostate) and leaves the Muslims." That's just one verse from this book concerning apostasy. There are many verses in both the Quran and the ahadith (plural of hadith) mandating death for various classes of people. It's also unlawful for them to make friends with either Christians or Jews.