Preds have sided with worthy prey when surrounded by enemies, they've also spared those that save them, if only once.
I could see Frank and a Pred slaughtering a large group of gangsters, with a meta-human gangster ambushing the Pred and Frank saving it, thereby causing it to pass up trying to kill him.
Frank sees it as just another right-minded thing prowling the streets, and it seeing him as worthy prey that gets a pass
Only later would Frank find out that it's basically a dick on safari and starts gunning for it, then it sees him as a real choice trophy and starts gunning for him.
In the end, the Pred gets Frank'd, Frank gets around its self-destruct, and the issue ends with Frank tear-assing through criminals with Predator blades, a cloaking device, and retooled plasma-caster
This must become a comic