Re: Hooray!
IMO, it's not so much that he started as a villain and got softened. Donald has always been a kind person at heart, and always had a sense of justice. But because his family (ie, Scrooge and Gladstone Gander) gave him so much pain, he grew extremely frustrated.
The first times he put on the mask and costume, he acted villain-ish simply as a way to get back at them, working out his frustration in the process. After that, with a cooler head, he went all "humm, I have all these cool weapons and tools, what should I do with them?", and started fighting crime instead :P
If you like Paperinik, I wholeheartedly recommend the fantastic PKNA series (49 issues + 3 "zero" issues + 4 specials; ignore the "ongoing" label, the series ended years ago). There was also a second series, PK2 (it lasted 19 issues), but it wasn't nearly as good (still decent, though). Finally, there was a third series that was actually a reboot, but it was terrible, so I suggest you avoid it.
I don't know if it has been translated into English, but if it has, it's definitely worth reading. I loved that comic so much, I was heartbroken when they decided to go with an idiotic reboot D: