I have had this issue for most of my life. My parents divorced when I was 10, and ever since then it's been a bit touchy. For years, all my dad could talk about was how horrible my mom is, so I finally completely cut off contact for a few years. Unfortunately, not long after getting back into contact with him, he turned that venom on me. Needless to say, an indefinite non-contact policy is in effect now.
My mom, on the other hand, isn't malevolent with her negativity, she's just patently aware of her age and is bitterly disappointed with where her life has gone. With her it's small doses, no more than an hour or two at a time usually, but I still see her fairly frequently.