Symantec had a network breach and had their source code stolen. If this happened to avast I would jump ship because by that point they couldn't be trusted with security.
I believe it wasn't their network, but rather one of their partners?
I would probably argue about the idea that something like that can fully be prevented if you just try hard enough (I mean, if one of the developers packs the code and gives/sells it someone... you can only prevent it by not having any developers or no source code
), and I also don't think that when it comes to antivirus applications, the bad guys can learn from the source code anything new they don't already know... but I'd be getting off topic here, so I won't