Ruthless means showing no mercy or compassion. If you really want to cut down on the clutter in your apartment, you can't be sentimental about keepsakes. You have to be ruthless and throw out everything that's not absolutely essential.
If you're ruthless, you're the polar opposite of Mother Theresa. You get called heartless and cold-blooded. That's one of the reasons the word crops up so often to describe dictators and tyrants, though it can also apply to other personality types — like corporate raiders and colleagues so success-driven that they'll gladly steamroll anyone to get ahead. It finds its way into milder contexts, too: Just be ruthless and rip off the Band-Aid already!