I would not recommend 100 yard kill on a coyote with a pellet gun. .17 HMR, .223, .243, 22-250 would be better choices
The ethics of hunting is always a matter of discussion and disagreement, and ultimately a personal decision. Coyotes have been killed with air rifles, as deer have been killed with a .22 RF, but that doesn't necessarily make either a good choice. There are some good rifle recommendations mentioned above, both air gun and firearm. I believe if one is to use an air rifle for coyotes, a HP .30 caliber and a range of 50 yards would be my personal limits.