Backstory; before she was CEO - she was a lawyer, and apparently a bad one because she was fired. But she didn't think she was a bad lawyer, said it must be because she's a woman - so she sued. AND LOST.
Then worse, she tried to extort money out of the company after losing to not sue AGAIN.
Then her husband, some kind of hedge fund guy - got caught in a fraud scandal. Which in combination with legal fees/costs from above, put her about -3million in the hole.
Then somehow, which is still unclear - she became Reddit CEO. And this is after people already knew who she was and hated her.
She takes over Reddit, starts trying to be a good CEO and monetize everything (make money for Reddit) - which in theory is good, but the way she pursued it was bad. She wanted to get more sponsors, but had a hard time making the sell with subreddits like /r/fappening and /r/fatpeoplehate, a whole bunch of other "hate" subreddits - so she banned them all, and started banning people who spoke out against her decision. This really inflamed the community.
/u/chooter AKA Victoria - was a Reddit employee responsible for handling all the AMA's - and coincidentally after a "disastrous" AMA from Reverend Jesse Jackson on Wednesday, she was fired for reasons unknown.
She was highly respected among the majority of the default subreddit moderators, who in turn used that opportunity to bolster the fact there have been several "lacks of communication" between Reddit and the moderators of said subreddits - and they decided to do a "blackout" - making all the default subreddits "Private"/unviewable for about a day - causing all sorts of shit; witchhunting, several petitions (one nearing 50k signatures for Ellen Pao's (Reddit CEO) to step down). - Basically Ellen Pao became the face of the problem.
During this "blackout" there have been several other damaging stories about Ellen Pao surfacing - most notably 1 today where a Reddit employee was fired for having cancer. He did an AMA, but when it became clear that his speaking out about this at all negated his severance package, which wasn't money but health coverage - Reddit has made it a point to apparently go after that as well. So not only did she fire a guy for having cancer, but now she's trying to strip away his healthcare.
And that's just what's circulating about Ellen Pao, and why people hate her so much. Personally, I don't really care either way. You can't hate someone for trying to do their job, even if they do it poorly. But regardless, people look at her like she's the face of the problem - and they want her gone. Hence this petition, that's already over 50k signatures.