Yeah, as I said, it was the warrior, who took diablo's soulstone with himself, but succumbed to evil and was possessed by Diablo. So, yeah, kinda turned evil. Still, at that point I wouldn't call that body the player from the first game.

Afaik originally in D2 we were supposed to kill him and that would be it, but the animator company decided that it would be cool to animate some dude piercing his forehead with a stone, and since there wasn't anymore dev time Blizzard North decided to go with it. That gave way to to justification for the corruption/possession of the D1 warrior character and thus the story of D2 and kinda D3.

Oh, btw, Blood Raven, the second quest you do in act 1, is the rogue from D1; and the summoner you encounter at the end of the arcane sanctuary, the one who had Horazon's journal, is the mage from D1.