I've read a number of theories about why people think he's guilty, but I've never read one yet that rings true. He was a writer and obviously an intelligent man, so I feel that if he was planning on killing her, then he would have really thought about the details. There are so many much easier ways to kill a person and get away with it.
For example:
He and his sons had money problems, so he decided to kill her like he killed his previous wife by pushing her down the stairs and making it look like an accident. But, this makes no sense to me as it's actually pretty hard to kill someone by pushing them down the stairs, so what would he have done is she had simply got up and run away?
Or, he hit her over the head with something (a blowpoke) and then pushed her down the stairs. But, again, it's actually quite hard to kill someone with a single blow to the head as the skull is so thick, and even 20 years ago, they had some forensics, so surely he would have considered this when he was planning on killing her?
Or she was lying at the bottom of the stairs dying and he held her down with his foot until she died of blood loss. But, what was his plan if she didn't die of blood loss or if someone walked into the house?
Or, maybe they had an argument at the top of the stairs and he spur of the moment pushed her down the stairs. I could see this happening but then all the stuff about his debts and his previous wife become irrelevant, so if he pushed her down the stairs and she hit her head, it's also possible she fell down the stairs.
I don't know, but if someone who thinks he is guilty could explain how he did it. I would be interested in hearing how. For example, the Helen Bailey killing. I can see how he planned it out, how he thought he'd get away with it. It makes sense to me. But, I can't see it with The Staircase killing.