Actually, when it comes to google, they don't use overbearing advertisements. And when it comes to advertisements on their apps, I've noticed little to none. I use google wave, google talk, google groups, the list could go on. And most of their advertisements on the applications are advertisements about other google products (which is neat in my opinion)
When you're asking if google will turn "evil", do you mean like sell all of our information to telemarketers or the government? No, probably not. They're a very large company with millions of people's trust worldwide. When you look at it that way, they will try hard not to screw up with our information and let it loose. Also, Google isn't run by idiots. They're not going to take our information and **** us with it, that's why they're still around.
So no, I don't think google is evil per say, they do have some negative things about them, although the pros outweigh the cons (in my opinion at least). For example, if you were to write a piece of work and save it in your google documents to show others, google could take that writing and take 'ownership' over it. That's the only *problem* I have with google, but if you're doing something simple as sending a document to fellow co-workers that is copyrighted already, then you don't have problems. Also, I'm not going to say that google does this every time, or that they have even done it yet. But if you're putting an article onto their servers, why can't they claim it as their own?
That's 1 con out of a thousand pros for google, so no, I don't think they will ever turn evil.
Hope this helps.