I think that is the hard part all of us have had to (or will have to) come to terms with. They're not bad people all the time. Heck, if they were, it would be easy to leave.
I remember reading once about an experiment on rats. They put a treat dispenser in the cage and administered treats in three different ways. One cage got a treat whenever they pressed a lever, one cage got treats at first and then they stopped being dispensed. The last cage was totally random. Sometimes they would get a treat, sometimes not. No predictable pattern.
The rats who got consistent treats were very casual about it, they went and got one when they wanted, no big deal. They didn't overindulge in the treats. The rats who got some at first and then no more stopped pressing the lever pretty quickly, they knew nothing was going to come out. But, the rats who got the intermittent treats became neurotic and obsessive. They pressed the lever over and over hoping for a treat. When they got one, they gobbled it down and went back to pressing the lever. They couldn't live their lives because they were so obsessed with the lever.
It's like that with the alcoholics in our lives. We keep trying, hoping for those crumbs of affection, love, kindness, respect, that only come once in a while. We go crazy pressing the lever. It becomes all we live for.
L