Hypocrisy is the worst.
My boss hired his worthless dipshit son. The kid had carte blanche for three and a half years: mouthing off, showing up late, hiding in the bathroom for an hour so he wouldn't have to do anything, taking naps on the clock, you name it. That cost my boss most of the respect I had for him. The clincher was one day, after his kid lied about being sick so he could watch football all day instead of coming to work, the boss told me "this situation and his attitude are not going to be tolerated." So, after four years of tolerating it, and forcing the rest of us to tolerate it, he lied right to my fucking face about how important the issue was. That was the last straw. Thankfully the boss lost his job soon after.
People who "just don't get it."
My uncle tries to convert people to Christianity all the time. Long rants about how "if you just look at the facts" you'll realize Jesus was the son of an all-powerful god who will judge us all someday. 'Look at the facts'? If he were simply to admit he can't prove anything, and believes because he chooses to, that would be totally fine. If he could just shut his goddamned mouth once in a while, that would also be fine. Instead he has to act like everyone who doesn't believe is an idiot for not following a nonexistent trail of breadcrumbs. He's missing the point. Faith isn't about facts. It's about believing when there are no facts. I've got a second religious nutjob uncle who thinks it is his personal mission to convert everyone on the planet, and that fear is the best way to do it. This man is never without pamphlets about hellfire, and always has something awful to say about gays ("sodomites" as he calls them). Why does a god of love and compassion espouse hatred so readily? Oh wait, that's just my idiot uncle and millions of others exactly like him! Way to set an example!
Some folks just can't make good choices.
A new girl at my job was smart, hard-working, reliable, pleasant, everything we want. I found out she was living at a homeless shelter because she was a former addict who had been screwed by life and herself. She was turning things around, had gotten clean, and this job was the next big step forward. She loved the job, we loved her. Things were going great. Then she quit her job to get back together with the abusive homeless druggie scumbag ex-boyfriend who had nearly killed her with drugs and violence. She didn't love him, she was terrified of him and the busload of scumbags to whom he owed money, she could see even a shelter was much better than sleeping in the woods in a cardboard box (which is where the two of them ended up), she had gotten through the hard work of withdrawal and was officially clean, she had found a steady source of income.....and she flushed it all away to go be homeless and abused some more. I simply do not understand bad judgment on that level. Might as well leap off a tall building, it will end the same and you'll suffer a lot less.