Right. Me too. The narcissists I grew up with, and am still related to, are only predictable in the sense that, at some point, they will betray you. It’s a given. It could be because they’re trying to “teach you a lesson”, or they’re annoyed at something (else, usually), or they’re bored and it’s something to do. Feeding their ego is never a control, because they don’t think they’re doing anything wrong when they betray you. The only “controllable” part I can think of would be keeping them busy attacking other people, and then sure – the cost of feeding that excludes everything else.
If you’re dealing with a narcissist and you think they’re not betraying you somehow, I would say there are two things going on: you’re either not aware of how you’re being undermined (you can be sure at the very least that they’re talking badly about you, behind your back, to everyone), or you’re inured to that kind of abuse and betrayal. Like, you don’t even consider it betrayal, because it’s normal to have someone talk to you like that (or do those things, etc.)