Why do dead bodies of animals/humans smell bad, while dead plants do not?

Decomposition of body begins at the moment of death, caused by two factors: autolysis, the breaking down of tissues by the body's own internal chemicals and enzymes, and putrefaction, the breakdown of tissues by bacteria. These processes release gases that are the chief source of the unmistakably putrid odor of decaying animal tissue.Decomposition of plant matter occurs in many stages. It begins with leaching by water; the most easily lost and soluble carbon compounds are liberated in this process. Another early process is physical breakup or fragmentation of the plant material into smaller bits which have greater surface area for microbial colonization and attack. 

During decomposition of animal carcass various chemicals such as ammonia, cadaverine, hydrogen sulfide, and putrescine are produced by the consumption of proteins by anerobic organisms which are characterized by their foul smell. However in the case of plants, due to a low protein content the extent of production of these foul smelling gases are very small. Therefore decomposition of plants are usually oudourless. Moreover the plants are primarily composed of cellulose and lignin, which are acted upon by very few microorganisms, which ultimately results in much delay in decomposition of the plant.

Why is it easier to tear paper along a crease?

Paper is made from wood, and wood is made from long fibers. Paper is made out of cellulose fiber material, and these fibers line up in a particular direction, during the manufacturing process.  There is a distribution of the fibre sizes,  the finer ones penetrate into the web of larger ones holding them together. Typically the manufacturing process leaves the fibers are more or less parallel. So it is easier to tear in the direction that separates fibers from neighboring fibers than in the direction that breaks fibers. Creasing paper breaks and/or separates some of the fibers, making it easier to break/separate the rest. Excessive weakening of the paper happens along the fold line because the fiber-to-fiber bonding along the fold gets loosened. Hence, once the tear is started, pulling the paper apart a little propagates the tip of the tear a little farther. Because of the loosened bonds, the pressure of tearing acts more effectively along this direction rather than getting diverted by the larger and stronger bonded fibres. So the tear follows the crease.