If you want something that runs in the background, automatically scans your downloads, and monitors your system, you’ll want the paid version. The free version is fine if you just want to check for and remove malware. If you want to remove malware, spyware, and other garbage software from your Mac, we recommend you download and run Malwarebyes. The Malwarebytes Premium features that will monitor your Mac for malware and spyware, prevent infections before they occur, and automatically download updates do cost money, but Malwarebytes does provide a 30-day free trial. Anyone can manually initiate a scan and remove malware with Malwarebytes for Mac without spending a penny.
The standard scanner that checks for malware on your system and removes it is free. There are now two versions of Malwarebytes for Mac, a free version and a premium version.