"Why wouldn't you travel by main roads?"

Episode 1, The End's Beginning - Renfri

Now, to somewhat more briefly talk about Renfri...

We have two stories told about Renfri, one from Stregobor who is an unreliable narrator but convinced enough of his rightness that it's likely mostly true and one from Renfri herself, who admits she has a motive of trying to convince Geralt she's not a monster. We also have one object, the brooch, which is mentioned in both stories and possessed by Renfri.

Stregobor introduces the topic saying Renfri is a monster.

"The worst kind. The human kind. Its name is Renfri."

He elaborates on the general traits he claims the girls possessed.

Stregobor: "I found horrendous internal mutations among them. I tried to cure them, locked them in towers for safekeeping, but the girls always died."
Geralt: "Internal mutations?"
Stregobor: "They were autopsied, of course, to confirm my suspicions."

So, as I said, Stregobor is not a reliable narrator. The sequence of events here: "I found internal mutations, then I tried to cure them and locked them up, but sadly they died...wait shit I mean I found out they had the mutations when I autopsied them after their tragic and unforeseen deaths." The only real question is if show!Stregobor killed the girls first because he was that confident or if, as book!Stregobor admits, he cut them open while they were alive.

I believe we can take it as fact the girls are mutants. Geralt is also a mutant. Later, we'll meet a gold dragon and it's stated the color means it's a mutant.

On to Renfri specifically:

Stregobor: "Daughter of King Fredefalk of Creyden. I delivered the princess myself in the middle of the afternoon in pitch black."
Geralt: "Under the Black Sun, so she's cursed."
Stregobor: "Do you consider me a fool, Witcher? Do you think I did not conduct research? Renfri was acutely affected. Her stepmother, Aridea, told me she tortured a canary, strangled two puppies, even gouged out her maid's eye with a comb."

"Do you think I did not conduct research?" Stregobor bellows in outrage, followed by, "Her stepmother, Aridea, told me," because that's apparently all the research you need to conduct.

Sorcerers, we'll learn in the next episodes, are usually placed at courts, and Stregobor delivering a princess sure suggests he was court wizard. Then either he stuck around and never personally saw Renfri do anything or he left immediately after. Given he admits "girls", plural, died in his keeping and that he believed the prophecy from the start, it's very likely he was forced to leave the court because he wanted to kill her or at least take her away right after her birth. After Renfri's mother dies her stepmother contacts him, at which point he still did not see Renfri do any of these things - it would appear he did not see Renfri at all, in fact. He doesn't say he tried to contact the maid who should've still been alive. He doesn't say he talked to anyone else in court about if they'd seen these things. He definitely doesn't mention so much as seeing Renfri from afar, and in fact it's unclear if he was even in the area or just set things in motion - if Renfri's father was not on board (and it sounds like he wasn't) Stregobor may have been banned from the country.

"I admit what happened next was not ideal, but with the lives of Aridea's own children on the line, we had to act."

Based on the claims of a stepmother who had children with a competing claim to the throne, he orders Renfri's death.

"So I dispatched someone to follow Renfri into the woods. We found him in the brush, Renfri's antique brooch jammed into his ear."

This is the first thing Stregobor can actually confirm she did, and it's someone he intended to kill her.