There is an old European folk tale that goes by many names, but is commonly known as the Farmer and the Viper. In the story, a farmer finds a baby viper dying from the cold winter weather. He brings the viper inside his home and nurses it back to health, upon which almost immediately after regaining his senses the viper bites the farmer and delivers a lethal dose of venom. Before the farmer dies, he asks the viper why he killed him after he helped him, and the viper responds that such is his nature- by nature he is a murderer, and the help given him did not matter because he acted in accordance with who he is.
So must this lesson be remembered with Islam. While there are many outwardly good people who are Muslims, never forget that the perfect model of Islamic piety for Muslims is a mass murderer and a rapist. Until they clearly and publicly reject Islam and embrace the One True Faith, they can never be truly trusted, for just like they asp, all it takes is an act of the will for them to embrace the evil nature that Islam gives them.