I don't buy that about her attitude. It's implied people either got permanently maimed or even killed due to her excesses. Even if it was a cover story, there's no need to go to that extent to make yourself hated.
Furthermore, he mentions her healing power wasn't great before. While we still don't have exact details how all these saint powers work, based on the current story up to this point it's safe to say certain acts and attitude does affect those powers. The old Jelly couldn't use her healing powers properly because she was a tyrantess.
The conversation with this mysterious guy seems to imply he doesnt know about the slime. He speaks as if he realizes poison failed to kill her when he dumped her in the forest, as he talks about her healing ability. But it did kill her, he was right.
If I had to speculate...and this is really a long shot take based on far too little info....he did do something to her again this time, as illustrated by the breaking sphere. My wild guess would be he cast a spell that causes amnesia. This would prevent a saintess from accessing her power, because we see that those powers can't just be intuitively accessed out of the blue. This didn't affect the slime part of her, but it prevents her from accessing her host memories.
Said spell was meant to leave jelly defenceless against the monsters attacking Celestria, which would lead to both being killed. He's not after her today, but he didn't say anything about the monsters.
The hole in this hypothesis is why didn't he cast this on jelly when he tried to kill her the first time? Though it's possible he actually did. But what he's saying now doesn't mesh well with the idea that he did. This manga is surprisingly detailed in some ways so I hold that as significant.
