My only problem with your theory Te Mirdala Mand'alor is why only "Dark Siders" are affect that way by Malachor. The sheer strain of resisting the "Dark Side" on Malachor is bound to have a draining affect that results in outward corruption of appearance.
I also would like to note that there have been plenty of heroes in Star Wars like Luke and Cade Skywalker who use passion to overcome the Sith without corrupting themselves. I disagree that Luke was falling, he utilized passion but with a degree of control that most Sith lack. Cade easily employs anger and hatred against the Sith, but is not affected by it all since he still tends to do the right thing. Revan used the emotion of love to redeem Bastila and drive him against Malak. Thus Passion cannot be considered the corrupting element.
Also, there is a different between seeking power and the means that you obtain it. The former is not necessarily evil, in fact it is the later that can be good or evil. Just because one has power or wants it does not mean one is evil. Otherwise we'd have Darth Washington, Darth Lincoln, and Darth Teddy as presidents.
Power is the capacity to enact change.