Well, the "insane" Genis-Vell was only really around for a little while before his title was cancelled. During that time, the stories involving him were a bit jumpy. So I'll do my best with a lowdown and I hope this helps you out. I'll try to be brief and if you want to know more you can either ask for more or get the comics.
 
He is the son of Mar-Vell, the origional Captain Marvel, and an Eternal from Titan known as Elysius. She had a fling with Mar-Vell and when he died she decided to have his kid using some cloning technology and some DNA. As a result Genis is much more powerful (stronger, faster, etc.) than his dad was. His power's include vast energy manipulation, super strength in the 100+ class, flight, breathing in space, faster than light travel, and cosmic awareness (and he also time traveled once). He inherited his dad's cosmic awareness (basically omniscience) which takes months of deep docus and meditation before he finally learns how to use it properly. He has a really deep desire to live up to his father's reputation which gives him some daddy issues, him always striving to be perfect and what not. 
 
The cosmic awareness drives him insane once he finally learns how to use it. He wants to save everyone, but all of his actions exist in shades of gray outcomes and he's needed everywhere at once. It's really the stress that gets him. Afterwards he decides it better to not care about life so much and he gets a little less human about his actions. He eventually starts believing himself to be a god and starts occupying himself with various shinanigans for entertainment. At this point he's basically a cosmic Deadpool. It's pretty funny and there's a lot of philisophical/ social commentary involved that I found really funny/interesting.  
 
There's also some scans of some of his funny rants here on comicvine if you''re interested.