I don't know if this will post since someone said this topic is closed, but on my old - very old - iMac 24", I had a complete system failure. I contacted Apple and the Apple guy said the problem was MacKeeper. He advised me to download Malwarebytes and took me through the steps.
Malwarebytes came via MacUpdate, which I wasn't aware of until I started getting e-mails from MacUpdate.
As it turned out, the problem with my old iMac was two-fold and had nothing whatever to do with MacKeeper: 1. the video card was failing (the machine, after all, was 9+ years old) and 2. the reason why my system was so slow was because I'd exceeded the processor's capacity when upgrading my O/S (Yosemite was the last O/S update).
A few more questions about the age of the computer and processor type would have at least solved the mystery of the second problem. I grant you that it's harder to assess a hardware problem over the phone, but I gave a clear, real-time account of what was happening on my computer and a real Apple technical person should have had some idea of what it was (I suspect he was working from a book and not from actual first hand knowledge).
I agree that MacKeeper ***** and I won't install it on my new computer. The Apple guy told me to remove MacKeeper (which I did but I don't remember how, since there was no uninstall option that I could find, as others have mentioned) and then, at his direction, I installed Malwarebytes (from Macupdate) - or tried to because at that point the video card failed entirely.
MacUpdate and MacKeeper are two separate entities.
And just to point this out, I'm on the MacUpdate site right now and there are no ad banners at all.