Pruning toxic relationships

For me personally, cutting ties from relationships that are one-sided and draining is strangely difficult. I'm a nurturer by nature, and I tend to want to "fix" everything and everyone. If I come across a flea-infested, three-legged dog who bites, rest assured I'll end up having to flea-bomb my house and get a tetanus shot. Same thing with people, the more dysfunctional they are, the more drawn to me they seem to be --or is it me drawn to them? Haven't figured that part out yet. I have my own homeless guy. When I see him, I say to my husband, "pull over, it's my homeless guy!" I hand him a little money, ask him if he needs something to eat, how he's doing, etc...he's always incredibly thankful and kind. He doesn't want to go to a shelter, he's a nomadic sort of man who seems to survive, if not thrive, on the fringes of society, but something about him draws me in every time I see him...I want to bring him home and prepare him a meal, make him take a bath, put on clean clothes, and comb his hair, then I'd fix up a soft, fluffy bed on the couch and pop in a DVD for him to watch.
Alternately, there's another homeless person in our city we call "Dan Dan the homeless man." He's a little on the mean side...enough for me to know to 'mostly' keep my distance. I once saw him sitting just inside a clump of shrubs on the side of a convenience store parking lot, so I got my husband to "accidentally" dropped a $20 bill while COUGHING loudly enough to get his attention. While we were driving away, we saw him emerge from the bushes and snatch the bill. I was warmed a little inside. Not too long after that, he showed up at the Humane Society where I was volunteering, and began throwing old pizza over the kennel fence, shouting that we were killing the dogs with the dog food. He was enraged and police had to be called, and I was told by an officer friend, in no uncertain terms, that Dan Dan was dangerous. I've completely avoided him from then on, but I did notice he'd befriended a dog. Maybe he's just not a people person.

Why, then, with my more personal acquaintances, can I not just walk away when I see that they're energy vampires? One of my goals for this summer is to not only keep my garden pruned of weeds, but my life pruned of toxic relationships.
As I began to cultivate this plan for a weed-free life, I questioned my own ability to be a good friend. Was I being cold by contemplating writing off people who may consider me a friend? Is it right to just begin cutting people out of our lives? How did I decide who should stay and who should go? Surely something so delicate should not just be based on a mood, which in me, changes more frequently than I care to admit. So I figured I would use the wise words of Ralph Waldo Emerson - "A friend is a person with whom I may be sincere. Before him, I may think aloud." as a means of whether someone was truly my "friend" or not.
When I began going through the list in my head of people I was questionable about, it became clear that I really already knew...but I mentally put each to the test anyway, just to be fair.
If you can't speak openly and honestly with someone, sharing how you truly feel, they're probably just an acquaintance, not a friend. Of course I don't mean that because you can't tell your BFF that she looks fat in those pink capri pants, you should cut ties. Honesty and tact should always go hand in hand, in my humble opinion. Yet when it comes to genuine feelings, opinions, thoughts, etc... I mean the substantive meat of who we are, how we feel and think...those deep topics that, sure, can promote heated debates, but not change the love and respect that true friends feel for one another. If someone is offended by who I am on the inside then I don't want them as a friend, and vice versa.
Hanging onto unhealthy relationships is draining and promotes negativity in our lives, and life is too short, sweet, and precious for that.