Each of them harbored their own ulterior motive and was wary of the other.
Tips
memory
鬼 (ghost/devil) + 胎 (fetus, what you carry inside) = literally 'a demon-fetus inside you' — a vivid metaphor for hidden bad intent. Almost always appears in the fixed phrases 心怀鬼胎 'harbor sinister designs' or 各怀鬼胎 'each plotting their own scheme'.
