Heather wasn't Hudson's secretary, she was his ex-boss's secretary. Regarding Pym's role, one angle from the comics that could make him the antagonist is that the size-changing was supposed to have affected his physical and mental health, IIRC. What I'm imagining is that Douglass's Pym could have used his powers as a SHIELD operative back in the day but was forced to give them up because of these concerns. When some new young guy gets ahold of his formula and starts using it, the unstable part of Pym's psyche manifests in the form of resentment / jealous rage. Janet could play into this too if she's Pym's wife and closer to Lang's age.