Bill Null is Zeus.
- He has ridiculous success with women, and Zeus is famous for his conquests (in universe, spawning an entire species by himself).
- Zeus is well known in mythology for taking many forms.
- He wanted Earth left alone by the other Gods. What better place for a sabbatical?
- He had specifically requested at least one Drylen artifact to be hidden on the planet - it would make sense he would want to keep an eye on it.
- He often happens to be around Null when the weird stuff starts happening, but seems unusually calm about it all.
- If he found out about the Nexus and the plans for it, it would be an excellent way of keeping an eye on and interfering with his children's machincations.