(I admit that I have no firsthand experience with terrorism.)
The second part of the question reads, “can Christians carry weapons to defend ourselves?”
Christians carrying weapons is an individual’s own decision. Is it a sin for a Christian to own a weapon? No. But should we be relying on that weapon to protect us? Should we trust our Lord to protect us, or should we trust our weapon for protection? How many terrorists would I need to eliminate before I would feel safe with my weapon as my protector? One? Five? Fifty? Our prayers should not be “please help me eliminate my enemies” but our prayers should be “please help me to be faithful and accept your will”.
The first part of the question “How/can [we] stop the terrorists for killing Christians…” is asking what “We Christians” can do to stop the terrorists. As Christians, we do nothing. This is not a religious issue, but a civil issue. The terrorists are trying to make this a Holy War, but it is not. This is not about Christians fighting Non-Christians. Jesus is not instructing Christians to eliminate opponents to his teachings. As Christians, we need to remember to hate the sin, and not the sinner. Matthew 5:44-45, “But I say to you, Love your enemies and pray for those who persecute you, so that you may be sons of your Father who is in heaven.”
It’s natural to feel fear when people are threatening you. However, Jesus taught, “…fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body.” (Matthew 10:28) Let us remember, that no matter what happens, God knows everything. Isaiah 46:9-10, “…I am God, and there is none like me, declaring the end from the beginning and from ancient times things not yet done, saying ‘My counsel shall stand, and I will accomplish all my purpose,’”
If you feel the need to take up arms and defend your city, (region, or country) then do so. But this will not be in the name of Christ. Remember, you would then be under militia/military rule. You will be subject to that militia/military leadership and direction. You will be under the command of your leader, and not Christ. This battle would not be in the name of Christ. Our Christian fight is to be faithful.
1 Timothy 6:12 “Fight the good fight of faith, lay hold on eternal life, whereunto thou art also called, and hast professed a good profession before many witnesses.”