My version of why Ron reacted so violently to Remus being a werewolf…
See, "version": I don't own it.

Tonks witnesses a much-needed conversation between Molly and Remus.

Two weeks before the start of term, Molly had given the kids and other helpers an afternoon off from cleaning. It was a blisteringly hot day and everyone had been a bit snappish at close quarters. Remus had retreated to the library and was reading a book with the window wide open and a few Ever-Circling Cooling Charms floating in the room.
Nearby, Tonks had challenged Arthur to a Gobstones match, using a set that currently ran on ice-cold water – it had several spells on it that turned anything else into some sort of poisonous black gunk. The loser had to untangle them.
Sirius was poking through a box of old games that the Gobstones set had come from.

As the hour struck three, Molly brought around a plate of biscuits and cakes and bottles of cold Butterbeer. "Mind if I join you?" she asked.

"Not at all, sit down," Remus spoke for them all, then almost wished he hadn't. Molly had been a bit odd in her behaviour towards him; one moment warmly welcoming, the next hesitant, almost cool. And Remus had a sneaking suspicion it fluctuated with the moon.

Arthur had advised him to "give her time"; Ron, overhearing, had told him "she just needs to realise you're not like the bad ones". So Remus had been polite and friendly always, and hidden signs of his
condition.

As they ate the food, the adults talked about insignificant things. Molly leafed through a Witch Weekly magazine, searching for a recipe she thought Tonks might like – or so she said.
Suddenly, she sighed and straightened. "Remus, I have something of an apology to make." she said.
Arthur smiled. Tonks exchanged a glance with Sirius. Remus blinked. "Apology?" he repeated.

Molly nodded. "Yes. I tend to be a bit biased against werewolves. I have to know them for a while first, before that changes." she said sadly, "You see, when I was fourteen, my favourite cousin – fifteen
years older than me – was attacked. I don't know who did it. We rallied found him, at first. Built a shed at the back of the garden, made sure it was safe... But it wasn't safe enough. Somehow, he got out. Somehow, his little son got out of the house. They met and..." she gulped.

"He killed him. His own son. Or at least, we – we think he did. I remember seeing more than one set of paw prints and the shed door looked as if it had been torn down from both sides, but... He had blood on his mouth and woke up – so close to him..."
Arthur wrapped him arms around her while the others stared. Remus' mind was reeling.
Molly went on, "He was shattered of course. His wife took their daughters and left, breaking contact with all of us completely. We kept watch on him, but we weren't careful enough. He was quiet... Too quiet. A week before the next full moon, he disappeared. I found him... He'd killed himself." she swallowed. "Two weeks later I started my fourth year at Hogwarts."

Remus sighed. "Oh Molly... Thank you for telling me. I don't suppose the original werewolf was part of the Crescent Movement?"
Molly scowled. "He left behind a piece of crescent-shaped silver, yes."
Remus nodded. "The Crescent Movement's biggest leader was Greyback. He got me after my father …repeated certain views of society out of anger." he swallowed, "I understand, Molly." he said. All too well, he thought. The cousin's situation was one of his biggest fears, and he could easily comprehend why the man had committed suicide.
Molly, not knowing the extent of his dark thoughts, smiled slightly. "So do I, now." she said.