Firstly, I'm new here so I apologise in advance if this is in the wrong section. Major spoilers ahead.
On to the topic, the scene after the final boss fight intrigued me. Correct me for any incorrect details as I am yet to physically play the game yet myself, but after viewing a few cutscenes, this one peeked my curiosity. Firstly, when Sebastian supposedly disconnects himself, there is still a surreal atmosphere about the setting (Giant brain and all, plus incorrect storage of Ruvik's brain), implying this part is still played out in the mindscape. Shortly after Ruvik appears and interestingly claims 'I created this world... You cannot keep me here' before disappearing. Due to being the original creator of STEM and obviously his mindscape, I'd presume he roughly knows all of the happenings that occur there so pulling one over his eyes would prove difficult. Also, despite being obviously after they both fought, both are in relatively stable condition (enough to retain consciousness and stand at least).
Now the interesting thing is, Ruvik is fully aware of Sebastian's intentions to stop him. I'd also presume due to his high intellect that he can put 2 + 2 together and conclude Sebastian intends to destroy his brain yet interestingly makes no moves or implications to stop him. One could take this as a (sorry for a late trigger warning) suicidal sense of escapism, OR another conclusion more likely. As they are still in STEM his brain was never physically destroyed... At least not by Sebastian if it WAS. Alongside this, with his claim 'You cannot keep me here' I'd presume he already knows he's got what he wanted and from the moment that he got Him, Castellano's and Kidman's fate (more so Castellano's) was already sealed. Saying this, he probably was already dawning this fact to Sebastian - a late warning to tell him it's too late. The destroying of the brain may also have been symbolic to Ruvik's freedom achieve, or at least part of it. Either way, he seems unconcerned as to what Castellanos has planned for it.
I could go on more the remaining scene but it may go on a little. Any thoughts?