Talk About a Silly Mistake
Dexter, whos been killing people for years. Id guess he probably has close to 100 kills. At least.
Why would Dexter set up his kill room, knowing that Deb also knew he would be there. I'm sure with everything that happened before hand, may have clouded his judgement. But that was just a plain dumb mistake. So, Deb walks in on him, while hes talking to Travis, then she watches him stab him in the chest.
In my opinion, this is how it might play out. But each thing that Dexter says, will also lead to more questions. Dexter will probably say, that travis kidnapped his son and was going to sacrifice him. And he just lost it and wanted to enact revenge. I doubt highly, he's going to go into detail about him being the bay harbor butcher and many others.
I do think think this whole Deb being in love with Dexter is a very bad idea. Dexter will not recipricate her love. I also don't think she is going to tell dexter her feelings. After what she just witnessed. But, as usual, dexter will talk his way out of it, like always. And Deb, wouldn't turn in her brother, no matter what. I just don't think she ever would. Even if she found out about all the other killings, I'm not sure she would turn him in.