A low-stability demon is more likely to be 'a bit erratic' than full-on crazy. Low stability is more recognisably related to ADD-ish symptoms (): inability to plan for the future, severe concentration problems and poor self-control.
By contrast, a very high stability tends to lead to self-absorption, overanalysis, unwillingness to act or commit to a risk, and aversion to noise and bother. High-stability demons are usually tranquil and gentle. They may not be.
A demon with high stability will live a long time but be unlikely to take risks or break habits. A demon with low stability will live a much shorter, more eventful life. (Be aware that this is a simplified explanation. There's more to lifespan than stability, and more to stability than lifespan.)
A demon approacing its projected maximum age will grow bored and listless, showing signs of ageing, and may eventually cease to exist.
But since demons rarely survive to die of 'old age', stability can (in simplified terms) be thought of as a probability: the probability that the daft creature will get itself killed, which approaches certainty as the demon approaches the end of its expected life.
A more unstable demon approaching the end of its natural 'lifespan' is more likely to become impulsive and change its mode of existence completely (think mid-life crisis!), effectively resetting the counter to zero. See Ishtar.
And all of this assumes the demon doesn't know what its own stability is. A conscious effort can change things.
Sounds like Achilles' choice: a long and uneventful life, or a short and exciting one that would be remembered for centuries to come. --Ree