You're completely correct, usually this behaviour stems from a distinct lack within a person; a lack of one or many things they cannot find the courage to address themselves..so they try to read others down to improve their feeling of self-worth all the while loathing themselves and inhibiting any potential gain that could make by applying negativity to their own life instead of positivity.
The best way to improve self esteem is to improve someone else's but people like I mention in this post have an inability to find the ownership for such things and so their lives remain small and insignificant.