Calling proprietary software evil is an opinion, and there are plenty of examples of evidence that proprietary software was created in ways that one could label as evil. Give it a while and there might be some revelation which will cause lots of people to go 'oh, that Stallman was such a visionary, calling proprietary software evil'.
Now on this particular aspect of Stallman's reasoning I find him hard to follow because that would mean a whole class of something is bad whereas I believe it should only apply to instances on a case-by-case basis. But I'm going to hedge my bets here and sit it out for the next decade or two (assuming I have that much time remaining) to see if he might not be on to something again that is still hard to see from where we are standing right now.
One way in which this could play out is that in order to avoid certain societal fates is to have nothing but open source for certain classes of application (for instance, voting computers, software in use by the government in general or software that is used to power network infrastructure).
Don't be too quick to judge, Stallman has been right more often than I'm comfortable with on some of his most 'extreme' views.