1) Don't sign DP unless you really mean it. A DP means commitment (at least a diplomatic commitment) so if you are not ready to for that commitment, don't sign one. There are other threads about this on this (and other) forum, where this is explained.
2) Don't sign DoF unless you really mean it, if you think that this AI can/must be dealt with in a near future or another better "friend" will deal with that AI, don't sign a DoF.
If you try to befriend the entire world, you probably end up being the one without a chair when the music stops. There are a lot of conflicts in the world so it is usually better to pick a side and try to use that side to your advantage.
The two most correct posts in this thread.I honestly wouldn't consider it broken. It's a question of commitment, and there's potential consquences to the commitment. That's how World War 1 started, even. Or the 7 years war.
Players need to concede their sense of entitlement. Maintaining good relationships with one civ may well mean having to accept bad relationships with other civ's. The fact that civs form cliques is not a bug, it's a feature.
Having said that, certainly a DoW on an unremitting warmonger should not incur a diplo hit, regardless of whether there was a defensive pact or not. Indeed, if it was a sufficiently warlike civ, the other AI civ's shouldn't mind it getting wiped off the map.