Sounds like he has a mental health issue - depression & who knows what else. Being someone who has enjoyed pretty good mental health I always had a very tough time understanding people like your family member. Then my own DD, who is beautiful, smart, funny, talented began to suffer from depression - and my world got rocked. I've learned SO much about mental health - some people are blessed with an emotional resilience and can "do life" easily. They can take on challenges without fear, they can lose a job, a love, their home and somehow bounce back. That's me. Nothing is a problem, I can survive anything. My father deserted our family when I was 13, my mom was disabled and ill for a couple of years needing spinal surgery, our family was on public assistance, etc. I've lost a job, went through divorce, lsot people I loved and I've always managed, had my tough times, but got through.
Now here my DD has both parents, all the emotional support she needs, she's tall, thin, smart, pretty, she does volunteer work with disabled children, she helps old ladies... From all accounts she should feel accomplished, well rounded, satisfied - yet she has this huge hole in her emotions. She's tried to hurt herself, she feels this sense of hopelessness, she feels this pain that no one can understand. There's a family history of depression on my husband's side of the family - and my DD drew the short straw and got that gene (her younger borther is the most easy -going kid on the earth). And it's a struggle. We've worked with talk-counselors and psychiatrists, she'd on medication and meets with a counselor on a regular basis so she can re-learn good coping skills and discard the old ones that she developed based on the faulty way her emotions work.
People like your relative probably smoke weed to self-medicate. The part of his brain that regulates emotions is not working properly and he doesn't know any other way of thinking and responding. He needs counseling and perhaps medication to address his brain's imbalances.
So - you don't need to stick around and listen to his continued verbal complaints - but I do urge you to stick by him and encourage him to get the care he needs. Most counties have low-cost mental health clinics and facilities that can be of some assistance. Try to look up depression online and see what kind of guidance you can find for family member & friends. I can tell you this - it's not easy and it's generally a long road with bumps and potholes. We have had our ups and downs, there are weeks that are very tough - and others that are great. But if this is a person you love, then do your best to guide that person to get help. If he gets the help he needs he will see progress - but progress that is best measured over weeks and months not days.
Good luck mama-sista.