Wow, I'm so glad for threads like this! Thanks guys. So to recap what I got out of it:
1) Not a bug: When you DOW an ex-friend because of a DP obligation, it makes sense that other civs get mad at you. You weren't wise in choosing your commitments.
It`s this. I only ever enter into a defensive pact if it suits me personally, if I can risk it or if I`m afraid a bigger enemy is about to start war with me. Always check what your Civ you want a pact with is doing with other Civs, whom they are at war with, or denouncing, etc.
A defensive pact means that YOU share the responsibility of any other Civ you have that pact when it starts war. If YOU start a war the CIV also has to immediately go to war besides you and share the blame. This is the agreement.
It makes total sense that if you immediately co-join a war as part of a defensive pact that someone else started then everyone sees you as the war monger too, or at the very least, helping the war . It doesn`t matter that you didn`t actually want the war. You still joined in that war right from the start, by association you are as guilty and that`s how the rest of the world sees it in Civ 5 and reality even.
The AI actually seems to understand this. I was in a constant war with Washington on and off. In a rare moment of peace, I noticed that France was right next to Washington but at peace, so i asked France for a defensive pact, knowing my next war with Washington would bring next door neighbour Napoleon in. Napoleon seemed to know this, refusing all my bribes to get that defensive pact, and wise he was to as well, for he really couldn`t afford a war with washington.
So be very, very careful whom you have a defensive pact with.