If you click on "More Info" it will most likely say something about "due to low installation volumes, we can't guarantee we know all about this programme".
Basically, because RuneMate isn't installed on 50 million computers around the world, Windows doesn't "recognise" it as a legitimate programme.
Which, to be honest with you, is a pretty unreliable malware detection method.
Just click ignore/allow/whitelist it in your settings and you'll be able to install it fine.