I am totally obsessed with this case and have changed my mind many times. But I've finally settled on guilty. If you go back to the first scene of The Staircase, when he is at the pool, recounting their final moments together he says "And that's the last time I saw her alive. Ah.. well no, she was alive.. " etc. I think that's a fairly major slip up.
Also - why all the paper towels? And why "My wife! She's fallen down the stairs!" REALLY? Because most people looking at that scene do NOT immediately think - ah yes, a tumble down the stairs. He himself had written about an intruder/robbery they'd had WHILST AT HOME with all the lights on and with cars on the forecourt. So SURELY, your first thought would be not 'oh clearly fallen down the stairs' but she's been attacked. And why do you say 'She's still breathing'. Why not "Oh my God, she's in a really really bad way - there's so much BLOOD" Why not mention the massive sticky red elephant in the room... the blood!!!!
I think they watched a film together. After that, she left to check her emails. And then didn't come back for quite a time. When he finally realised what was happening, he went to see her and found her incadescent with rage and hurt. She said something like "everyone is going to know what you really are" and he snapped. At first he grabbed her by the throat but she pulled away, trying to dash up the back stairs. He pulled her down and slammed her head once into the skirting board and twice against a step. She passed out and he stood there, horrified. He was certain he'd killed her and went out onto the patio, possibly even down to the pool, pacing and working out what to do - his son would be coming back soon!!!
But he'd seen something like this before - even if he didn't do it - people fall down stairs. So he dashed in and opened another bottle of wine - pouring it down the sink and putting two glasses out. (The cops would smell the alcohol down the sink and note only his finger prints were on the glasses), Then he heads back to the scene and OH MY GOD. Not only is the blood horrific - she's been bleeding out all this time.. SHE'S GETTING UP. Her feet are drenched in her own blood and she's moaning - so he grabs... (what, I don't know - possibly a full coke can) and hits her hard, three or four times, until she's down again.
This is getting really out of control. He has to get rid of the emails. He goes through to the computer and spends sometime trying to hide evidence. He goes outside and buries the item he bludgeoned her with in some woodland. Then he comes running back and decides he'll have to clean up some of the blood - this is way to much blood for her to have fallen down the stairs, even he can see that. But then he's trying to soak it up and there's just so much and he realises it's going to look dodgy if he tries to dispose of the blood soaked paper towels as he wouldn't that if he was trying to get her help. So he abandons the towels - realising he can't carry them anywhere, they'll just have to stay there. First he tries to clean his bloody foot prints up from where he's gone through to the laundry - he takes his shoes off. Christ, what a mess.
After what feels like only 10 minutes, but has probably been over an hour, he steels himself to make the call. He knows people must think he found her ALIVE. And so he calls...
"My wife... she's still breathing".
That's my take on it anyway.