I agree. Treachery, when discovered, should certainly count diplomatically. Whoever you betrayed should hate you. However, their enemies may be positively inclined that you have so neatly stitiched up their opponents, though you'd still be untrustworthy.
Good civs like Elohim should in my opinion value trustworthiness more than temporary rivalries, so they should get more untrustworthy penalty and less fools of enemies bonus.
Balseraphs on the other hand, insane and looking for amusement->huge bonus for giving them laughs at others' gullibility.
And the betrayed's friends should also be a bit more upset.
And of course it should be the traitor who is counted as the declarer of the war