I don't think that having analysed the blood splatter, it was planted at all, the pattern doesn't show that. You can see there are two sides to it and a large point at the end, almost in a V shape formation. If it was thrown in two stages to create that effect,it crossed over at a point a little. Still, it doesn't its all created in one go.
This pattern suggests that someone was hit with some sort of blunt force at the front door, and we know that it's Nancy's blood. I would suggest that perhaps, quite simply, Nancy was pulled out of the front door and possibly pistol-whipped with the gun. As she's an older lady, her skin is much thinnner so it's unfortunate that she would bleed and bruise much easier than a younger person.
As for who or what or why and all the other speculations, who knows, but let's just look at the evidence there is, not what there could be.