When the Witness speaks to the Traveler before cutting the triangular portal into it, it sounds like there is legit sympathy in his voice for the Traveler's situation and existence. I think the Witness takes no enjoyment in his crusade and views itself as a pawn of the Winnower and also views the Traveler as a pawn of the Gardener. I think he hates the universe he stuck in and it's cruelty and wants to ultimately destroy it. When he enters the portal, I think this is where he goes to "commune" with the Traveler, which is essentially communicating on a deep or spiritual level. I think the Witness is trying to get the Traveler to see his point of view. I think the next step is to "drink the light". The question is does the Witness want to drink the Traveler's light or the Gardener's. If the Witness could bleed the Gardener until it died, I theorize that reality would start to unravel. The last step is to "kill" something. I think the Winnower can't exist without the Gardener and vice versa. If there is no light no shadow can be cast. If the Gardener dies I think the Winnower might die with it. If the Witness can destroy the source of the cosmos, which he views as full of cruelty as pain, then the cosmos will cease to be. I don't think the Final Shape is a shape at all, I think it is absolute nonexistence. No more life, no more death, no more pain. Just oblivion and nothingness.