I had planned to write this earlier and I saw the Pastor Hall at Pulpit & Pen had written an essay on the exact topic (linked below). So I'll keep mine brief.
What the terror group ISIS is doing is undoubtedly evil. No person, village, or region should have to suffer the tortures and terrors perpetrated upon them like the Islamic ISIS maniacs have done. I want to say, and this will likely be unpopular, but the 21 Egyptian Coptics that were beheaded recently were not Christian. Copts are not Christian. They are not martyrs for the faith, they will not be resurrected to a glorified body in heaven, and Revelation 20:4 does not apply to them.
Copts have popes, believe verbal confession to priests is necessary for the forgiveness of sins, worship icons and idols, are highly sacramental and ritualistic, pray to dead "saints" asking for their intercession of the living, believe in apostolic succession (from John Mark), have a works based theology of salvation, and more. Orthodox Coptics are a split-off from Roman Catholicism. Again, Coptics are not Christians.
There is an outline of what Copts believe here.
What happened to the 21 Egyptian men on the beach is horrific, but please stand on the Gospel and remember that Copts are an unreached people group needing evangelism, not veneration as brother martyrs.
Coptic Christians Not Christians, Evangelical Leaders Need reminding
Coptic beliefs from Encyclopedia Coptica