See my latest posting in the TDM-Modpack politics in really big game thread. I summarize the politics of the AI at turn 60. Basically some races are Murderous and some are Brotherly, while others get stuck at some mid-range point like the Sergetti. You can keep a trade pact going with even a Murderous ally by sending a message every turn trading resources. This keeps the AI too busy to send a break treaty message. Once your empire has matured so that your trade pacts are at maximum and you are generating the most resources you can stop sending Messages. They will no longer try to break with you due to poor trade since you have become an important part of their economy.

Several races will turn Murderous immediately after you meet them or a trade pact is established. Just trade with them. Some will improve over time from Murderous while others will get stuck there. Do not worry too much about it.