Muhammad started peacefully in Mecca, but when those he preached to mocked him willingly took up the sword to raid, rob and kill those who had mocked him and refused to follow him. His followers likewise killed non-believers at their prophets bidding.
Christian martyrs were humble believers who were willing to die for what they believed. They believed and taught the Gospel of Christ even when they were threatened with torture and death if they claimed the Lordship of Christ and denied Caesar was lord.
Christian martyrs were willing to humbly give their lives for the Faith.
The followers of Muhammad were, from the beginning willing to kill and assassinate the enemies of their prophet. A Muslim martyr is one who dies killing the enemies of Islam. Muslim Martyrs died with bloody swords in their hands.
That is a small part of the difference between Christ and Muhammad and Christianity and Islam.