Things I ought to do vs. Things I actually do.

When you decide that a friendship is over, perhaps it would be best to actually confront this friend and let them know why you've decided to cut ties. I've done this in the past. But usually it just ends with me apologizing and perpetuating friendships that cause pain to both parties. If saying goodbye only hurts one person, then that, by utility, is the best choice. You don't have to explain this. Just do it.