Cut 2 small triangles from the top corners of a rectangular graham cracker using sharp kitchen scissors. Cut 2 long, thin triangles from the bottom of the graham cracker to create a coffin shape. Repeat with remaining graham crackers using the first one as a template.
Lay 6 graham crackers on a plate. Draw crosses and the letters "RIP" on the center of each coffin lid using the black decorating gel.
Break 3 dark chocolate squares in half. Leave remaining 12 squares whole.
Lay remaining 6 graham crackers on a baking sheet. Place 2 1/2 squares of chocolate on each, leaving room on for the skull. Arrange 11 miniature marshmallows on top of the chocolate in the shape of a skeleton: 4 for arms, 5 for the body, and 2 for legs.
Set oven rack about 6 inches from the heat source and preheat the oven's broiler on the low setting.
Place graham cracker coffins under the broiler until marshmallows bring to color, 10 to 20 seconds.
Place skull candies above the marshmallow skeletons. Drizzle red icing over the s'mores. Sandwich the s'mores with the decorated graham cracker coffin lids.