The most secure way to use the software without paying is through the official Malwarebytes Free edition. While it lacks the proactive "always-on" features of the Premium version, it remains one of the best tools for cleaning an already infected PC.
: On-demand scanner that removes existing malware, adware, and potentially unwanted programs (PUPs). malwarebytes keys free
Using legitimate Malwarebytes licenses is crucial to ensure you receive: The most secure way to use the software