Hello micarusb1,
Let me put it like that... MacKeeper is a hot subject, you either love it or hate it. But technically, MacKepeer is not a malware. If you'd get annoyed by pop-ups that remind you about non-finished business with MacKeeper, it is not the reason to call it malware. After all, it's your call.
Officially, MacKeeper is a maintenance program for Mac that claims to fix a lot of issues on you Mac and optimize it, plus, on the top of that, it has live support feature (called Human Inside), anti-theft feature and anti-virus licensed by Avira.
MacKeeper is a bundle. If you are a Mac novice, a grand beginner, that could simplify your life a bit. So, you'll have one click instead of two.
Worth to notice that most of those features already exist by default on your Mac, minus few. It's just that they come under one GUI with a sleek interface.
Yes, they use controversial marketing techniques to promote the application which is part of the reason a lot of users have said not to download it. There have also been some user reports of slowdowns and deleted data. But, again, I've actually asked this question of a lot of security folks in the past, and they say it's definitively not malware.
BTW, you might have heard the recent news regarding Wirelurker virus proving that OS X is not THAT AV protected.