Is MacKeeper malware?

MacDefender and MacKeeper are two entirely different programs. MacKeeper does have certain qualities of malware, such as disrupting computer operations, but whether it's malware or not, it should still be avoided. It is a complete scam.

http://www.thesafemac.com/ongoing-mackeeper-fraud/

No. It's usable software, however, it nags. Furthermore, it's marketing methods are less than acceptable. It can prove difficult to uninstall, and, most importantly, it is not necessary any more than other similar software. Save your time and your money. Such "cleaning" software is not needed.

It's garbage.

I agree that it's not something most people want. I would suggest that you visit Macworld.com and do a search for "mackeeper". I'm betting the praise vs. complaint ratio will be very one sided.