While I'm not generally a big fan of Dan Abnett's writing, Cherubael seemed in character for a daemon.
No, Cherubael didn't have to save Eisenhorn... but daemons are timeless, and what they do might not make sense now, but may slot into some plan that they've yet to even think of. Besides, I think Cherubael did actually like Eisenhorn, despite constantly interfering - that kind of arrogance is very suited to daemons, with them looking on others as both admirable and inferior at the same time.
Obviously, he/it was angry when imprisoned and vengeful when released - but that sort of makes sense. Daemons are creatures of emotion (literally), so I'm sure they could easily get caught up in a moment rather than their long term plans. (People certainly can).
It's probably open to interpretation, but it's certainly not proof positive that the personality of the host can affect the daemon.