Well, I typically make alliances or stab based entirely on what I believe will be more conducive to my success. There are appropriate times for either, and it is useful to know whether or not your opponents would be better off without you, because that is a good indicator of their imminent stab.
I'm generally quite forgiving of people who stab me, as long as they are willing to work it out, because I think there are too many people here with the strike to kill mentality as opposed to the strike to wound mentality. As soon as they set their targets on a player, they cannot see the reason in backing off, when a rational look at the situation might call on a reversal of forces, in order to shift the balance of power, so that you end up significantly dominant in your theater.
As an example, France, Germany, and England are often seen as the northern/western theater of the game, and their triangular political positions make strike to kill much less effective. If for example, France and England band together to attack Germany, when Germany has been eliminated, France and England will be more or less equal in power, and very likely to fight amongst themselves simply because England cannot possibly hope to win the game without attacking France at some point. But a smart England, or France, will understand this, and before Germany is completely eliminated will coerce the Germans with the tantalizing prospect of survival, and turn against the other. The end result is that Germany's new ally will be stronger than both Germany and their opponent in the balance of power, and will also have Germany's gratitude. This is the strike to wound.
There's a reason that animals don't often fight to kill other animals in nature. If A, B, and C are competing, if A kills B, then he has just saved C a lot of effort, whereas if A just beats B, then B will understand where he is in the hierarchy, and will survive to compete with C as well.