all the women in his life are more powerful than he is, so that dark side of him never gets a chance to come out.
That's a good point. I guess we see a bit of it in 'The Zeppo' - and he can be incredibly cruel ('Seeing Red', 'Selfless') when hurt. It's more of a dormant thing I suppose... although maybe one reason he's often so nasty to Spike is that Spike's someone 'lower' than him - someone he has power over. Hmmm, interesting.