Sometimes a specific gospel writer might hone in on one part of the story without denying another part that he for whatever reason chose not to mention. You have to give the gospel writers some leeway there. In Matthew 28 it looks like Matthew just focuses on the one angel that the natural flow of his narrative dictated that he mention. It was easiest just to mention this one and what he said and then move on with the narrative, rather than get into the fine detail regarding the number of appearing angels, etc. Matthew had just mentioned the angel that rolled away the stone (an event that the other 3 gospel writers did not think to include) and in so doing just focused on the words of that one to the women who had approached the tomb. In so doing Matthew is not denying that there were other angels there as well. Again, the flow of his narrative would have made it cumbersome and unnecessary to go into that fine detail, detail that would not have served his narrative at all, would rather have bogged it down. So, there is no contradiction here. In Mark 16 Mark tells you where this angel mentioned also by Matthew was sitting – he was inside the tomb, on the right, dressed in white (as Matthew recorded in 28:3). Sounds like he appeared in a more intentionally frightening fashion to the guards (with an “appearance…like lightning” Matt 28:3) causing them to pass out. But now, he seems more benign to the women, just as a young man in appearance now, seated, non-threatening. The location – on the right – is significant, b/c it seems to imply the presence of another angel without explicitly mentioning him. In Luke 24 Luke describes the 2nd angel and has both of them talking to the women, saying things similar to that as recounted in Matthew and Mark. Mark’s reference to the angel “sitting at the right” seems to imply the presence of this other angel mentioned in Luke, who would have been therefore in the tomb but on the left. Still, no contradiction, actually all accounts consistent with each other even where one might leave out detail (the 2nd angel) that the other includes, and in doing that they still manage to agree rather than conflict.