Near the end Marius has a dream/vision of Tal Rasha with Baal being bound into him -things he's never seen before which happened ages ago and awakes from the Nightmare.
Dark Wanderer/Leoric's son says "Now you know what I seek Marius." without asking what he saw, and says "It is my brother. Sleep now, we set out with the dawn."
So what I'm wondering his
1. Dark Wanderer via the ending cinematic of D1 was heading for the east seeking salvation and ancient secrets to free him, BUT at THIS point...was he still Dark Wanderer from Tristram OR completely Diablo in a flesh suit? Did he slip up by mentioning that or did he actually give Marius the vision? Marius himself remarked "But these were not my own."
2. This is a big one here, prying into the nature of demons. Given then are big deep wells of hate and anger, by the voice acting and the expressions on the Wanderer/diablo's face...DO you think he cared about his brother the way that some mortal beings do? I think maybe he did, and human faces and voices are much more adept at conveying emotion than monstrous ones.
3.Was Marius too dumb to live, by not realizing he WASNT Tal Rasha's brother, but Baal's? OR was he thinking he was Tyrael's brother? Indeed I wonder what he was thinking during this whole adventure up until he abandoned the portal; did Diablo/Wanderer compel him to follow?
EDIT: I'm rewatching the first cinematic now, and I note that Marius says "Why do things happen as they do in dreams? All I know is that when he beckoned, I had to follow him."
4. Given Marius instrumental role in messing things up...did Diablo KNOW that Tyrael would be there waiting for him, and intend Marius to fulfil the role he did? You may recall Baal's line "You did exactly as you were meant to do."