Sirhan Sirhan is generally said to be an Arab Christian. Do you have evidence to the contrary?
He was born and raised in a Palstinain home whose parents professed Christianity, but people familiar with them at the time indicated that their family life was more akin to a muslim home. In addition he was schooled, and radicalized through Jordanian textbooks that demonized Jews as "animals," and called to "liberate" the Muslim holy places and Muslim lands. He said himself that he was inspired by a teacher who taught him about the Arab warrior Saladin.
So he may have been born into a Christian home...one that apparently operated very similarly to a Muslim home at the time...but he IMHO he was radicalized by Islamics teachings and ended up acting out those teachings just like an Islamic terrorist.