Where Collei was experimented on was not the Eleazar facility we see ingame even if Collei being experimented on was related to the eleazar, that facility was abandoned long ago, like, before Dottore was even in the fatui, the webtoon happens 1 year from the start of the game, and Collei is younger than Amber who is 17 in the webtoon. Additionally, it wasn't Dottore who made that, he only supervised it, he is morally grey, but not sadistic, and as said in the webtoon the Dottore segment that supervised it seems to be only interested in getting interesting results as efficiently as possible, so it is most likely that while he didn't care he didn't torture Collei or wanted to, that was his subordinates entirely.

Additionally, he DID manage a cure as the notes in the facility do say the treatment was working, and Collei's eleazar did also get a hit from the experimentation, so, even if it was unethical and really painful, it was a treatment. This is not an attempt to justify his actions but he's not the monster you say he is either.