The first thing that came to mind instantly before i was finished reading the first sentence of the dream part is that the rattlesnakes somehow represent all the negativity around you and all the people who you feel are either against you, or that oppose you in your life. As I continue to read, and finally get to the end, the only thing that I can see from this is that it shows how you feel and felt about what you've been through in your past, and then you finally end up at the point you are now; in a state of "darkness". So, you are you, the snakes are all the "attacks" on you, and being bit would be the point of no return where the game completely changes. It's currently dark because you're stuck there, not seeing what's around, and not knowing if you can recover from what has happened. The dog, is your only defender, through this, so either that's a real dog (if you have that dog) or it represents someone who has always been there helping to shield you from the other people who are constantly at you, attacking you. You keep having the dream, because you are stuck at this point, unable to move forward. This dream in no way means your end, your demise, your death. It represents your situation up to the point of the main attack that set you in the place your in now, mentally, and physically. So, although dark things are happening in the dream, I wouldn't say this is necessarily a dark dream.