This is an interesting concept. It gets me thinking: what exactly makes this "slightly evil"?

It's not actually doing anything nefarious. It checks to see if it can use your credentials to access certain sites, but it doesn't actually do anything with them besides tell you that it worked.

There is of course the potential for doing nefarious things using your credentials on other sites. But... all software which receives a username and password has that potential.

I can only conclude that it's "slightly evil" to point out the possibility.