Glad you've changed your mind, Patchou.
Well... I personally believe that while releasing the source code will undoubtedly mean problems such as people not giving appropriate credit, fake installers with trojans, among a host of other problems, I also believe that releasing the source community will yield some great benefits to the entire Messenger community. Sure, I understand that people will be redistributing it with viruses, and that will undoubtedly be easy to do. However that is already perfectly possible, so I don't see much difference with respect to that. The users have to be well-educated to only download software from the official site or authorised mirrors. And to run a virus scanner, obviously.
Possibly the most important thing to decide is on the license to use, or whether to create your own. But really, do what you think is best for the community and yourself, Patchou, weighing the pros and the cons appropriately. I'm sure you'll never be forgotten whichever way you pick.
Pedro.