This is something I've very much experienced and observed. I've got a few close relatives/former friends like this - they seem unhappy in themselves and really try and drag other people down to their level.
They tend to target others who themselves lack confidence and try and keep these people 'in their place' iyswim. They don't like it when these people in some way 'better themselves' e.g. if they lose weight. get fit, get a new relationship etc.
These people are real drains