The definition of a Christian martyr is one who dies for the sake of his/her faith that Christ Jesus is indeed God Incarnate and One in the Holy Trinity with the Father and the Holy Spirit.
1) To be considered a Christian martyr, one would have to be a believer (Matthew 5:11,12).
2) If a Christian dies naturally, he/she would not be considered a martyr, but perhaps may be considered a confessor or a righteous person, depending upon the circumstances.
3) Every person should be prepared by living a life of repentance through the Holy Sacraments, in addition to expressing brotherly love and charity towards others. If one chooses to not live his/her life in this manner, then, in essence, one is choosing his/her destiny.
4) Only God knows the depth of one's heart and will judge each person according to His mercy and His justice.