Some ancient Christian writers say that Simon and Jude went together as missionaries to Persia, and were martyred there. If this is true, it explains, to some extent, our lack of historical information on them and also why they are usually put together.
Simon is not mentioned by name in the New Testament except on these lists. Some modern writers have used his surname as the basis for conjectures associating him, and through him Jesus and all His original followers, with the Zealot movement described by Josephus, a Jewish independence movement devoted to assassination and violent insurrection. However, there were many movements that were called Zealot, not all alike, and Josephus tells us (Jewish War 4,3,9) that the movement he is describing did not arise until shortly before the destruction of the Temple in 70 AD.