Why is the installer quitting? How do I get rid of Malware on an iMac?

I believe I have some malware on my computer. I was trying to download something from a site I have always downloaded stuff from in the past so I thought it was safe but when it told me my flash player was outdated and I needed to upgrade it, that should have been the red flag right there.

I was told about a free application called Malwarebytes that is supposed to be really good at locating and getting rid of malware. I downloaded it but here is the next issue and I don't know if they Malware is causing this to happen or not but when I opened the .dmg package, the installer quits unexpectedly every time. I cannot even install it. Is there any other way I can try to install the software so I can run a scan?

If anyone has any other solutions, please list some suggestions.

My system drive is partitioned - 10.8.5 on one side and I have 10.12.6 on the other side. The Sierra side of the drive seems to be okay, its the 10.8.5 side that I believe has the malware.