But, on first playthrough, due to the lack of explanation, I was under the impression that the Gurs were more like such nomadic bandits and attacked him as a representative of the law, or just an overly rich guy who wandered into the wrong neighborhood.
I think it's definitely easy to miss, but he can talk about how he was killed by them because of one of his rulings, and the fact he refuses to elaborate is very suspicious. It does sound like Cazador knew him and maybe had something to do with it as well, though, because of how suspiciously he brushes off that Cazador happened to be there.
Here are his different lines talking about it:
"I was attacked. A gang of vagrants, a tribe of wandering 'Gur', took issue with a ruling I'd made.
They beat me to death's door when Cazador appeared. He chased them off and offered to save me. To give me eternal life."
"Not him, no. A gang of thugs attacked me, angry about a ruling that I'd handed down as magistrate"Going through these lines reminded me of another contradiction, albeit smaller than others- or maybe I misunderstood. Both in EA and the current game Astarion can talk about Cazador inviting him to dine with him, serving him a putrid rat. I assumed that he'd be in the room where Cazador is feeding. But then in Act 3 Ulma assumes -and is never corrected- that Astarion has never seen Cazador feed from his victims.