The Greek martys is a court word: a witness, someone who testifies to what they saw. In the New Testament it still carries that plain sense, and the apostles are witnesses of the resurrection in the ordinary way that anyone is a witness of anything. The narrowing to someone who dies for the testimony happens in the second century, around the deaths at Smyrna and Lyons — the word acquires its blood after the fact. When this dataset marks a person a martyr it means the later sense.
The Early Church · ch. 2