When someone really tweaks a nerve with you, saying something heinously insulting, or throwing a wrench into your plans, or questioning your taste in wedding plans...or your groom, you're probably going to boil up in anger and want to lash out. They do say it's better to get anger out than suppress it, right?
Well, reacting in anger isn't going to get you anywhere. You just sink to their level and make them very very happy. Because their goal was to pop your happiness bubble and make you miserable like they are.
So for now, write out the best tell-off letter you can -- by hand, so there's no file for anyone to see later -- and then toss it away. Or burn it. I just wrote out one of those babies today, and I feel a lot better. It helps so much to get that vitriol out of your mind, onto paper, and into the trash. For really bad offenders, you might find yourself scribbling out several notes over the course of some time.
Face it, anyone who's an adult is a finished project. You're not going to teach them anything by telling them a better way to handle their misery. You're not going to touch upon any sense of compassion in them, because there probably is none. So don't waste your time trying to teach a bottom feeder to become a benevolent soul. It's not going to happen.
Hang out with your benevolent soul friends, and absorb the good vibes of their input and their company. Leave your misery-soaked acquaintances to their own paths. This time is too short to let them have any effect on your fabulous life.