Because manipulating and attempting to kill Baam could be justified after Rachel's reasons are revealed, I'll leave that out of my Rachel analysis.
Regardless, I can't stand Rachel not because of what she did to Baam, but because she is a nasty human being. The following explanation should explain my reasoning in full: Out of spite, she stabbed Dan's legs. Then she pretended to be injured and milked it for a pretty long time. While ascending several floors of the anime's storied tower, she never attempted to contribute or assist her team in any way. She has never attempted to improve herself all the while. Plus she refuses to accept responsibility for her naughty behavior. She expects her teammates to treat her with respect and affability while offering nothing to them whatsoever and wailing like a spoiled kid.
Griffith from Berserk is a good example of a far more likable version of Rachel. Both are willing to go to any length to achieve their lofty dream. Griffith was willing to subject hundreds of thousands of his comrades to the most heinous mutilation and death, then force his best friend Guts to watch him commit an unspeakable act to Guts' pregnant fiancee.
Despite his far more heinous deeds, Griffith is a far more compelling villain due to his numerous redemptive characteristics. Because he worked exceedingly hard to better himself and his squad in order to attain his goal, he is personable, a master tactician, and a great combatant. Rachel isn't personable, tactically astute, or competent, and she makes no attempt to be. Instead of working to better herself or her team, she fabricates injuries and lies about them. How can you not classify her as one of the most hated anime characters? She possesses no redeeming traits whatsoever.