P.G.
You feel guilty because we are "supposed" to feel a certain way about our parents. But just because he married your mother, that didn't automatically make him your dad, and his actions certainly don't give him that honor either. Biology doesn't even make a parent - love and responsibility do. I speak to my bio-mother about 3-4 times per year, so I know where you're coming from.
Don't bother with him. Cut the ties. Move on. You'll feel less guilty as time goes by. If you don't have to be around him, you probably won't have to tell your kids much anyway. And if they're exposed to him at a family gathering somewhere down the road, his behavior will show them why he's not part of your family. Good luck!