My personal opinion is that if it isn't in the game or something officially released by Larian tied into the released game not EA, then it can't be canon.
And also that it leaves it entirely up to the player and how they want to view him (which is probably why Larian are being intentionally vague).
I also don't think it matters, if he was evil before it would have been for what 15 years or so? He was supposed to be in his mid 30's when turned I believe, young for an elf, plus he wouldn't have walked into a job with that much power as a teenager. I reckon anyone completely controlled, tortured and abused for nearly 200 years has repaid any possible debt to society accrued from what they may or may not have done as a young adult many years previously.
My personal head canon is that he was probably completely oblivious to any ramifications handed down as judgements, gaily continuing his merry way drinking the taverns dry, playing dice and visiting brothels, and clothing shops and buying jewellery for himself and the job was just something he wanted for the intrinsic status it gave him. He might well have taken the odd bribe here or there if it gave him the illusion of being powerful but I don't think he was evil, more heedless and, at the end of the day, very young.
I wouldn't mind in the slightest if he was outright evil in those years either, its just not my normal head canon on him.
But thats the benefit of Larian not giving us a ton of canon information we can all imagine him exactly as we want him to be
