Can someone please, please explain to me why Hermione is "evil" because of her actions in OotP?

Yeah. Her morals were questionable at best. She put someone's life at severe risk. She completely humiliated one of her peers.

But how does this make her evil? Are we just supposed to completely ignore the scenarios surrounding these choices?

Yes, she did lead Umbridge into the forest, where she knew the centaurs would (to put it bluntly) finish her off, or at least keep her busy. However, I don't think she woke up that morning and thought "Hmmm, Umbridge has gone too far. Centaurs would be pissed off if she entered the forest, wouldn't they?"

I think it was more along the lines of 'No wand. Captured by seemingly psychotic student body. Interrogation by Frog-Face. She's going to torture Harry, this is all kinds of wrong. This is bad, this is very very bad...' "I know where Dumbledore's weapon is!"

What the fuck was she supposed to do? Use her l33t ninja powers to escape from Millicent's grasp, shatter Umbridge's wand with her SUPERIOR WANDLESS MAGICK, strip down to a leather bikini and heal everyone's souls with her magical tears?
And then make the sweet lurve with Draco because they're secretly soul-mates, right there in Umbridge's office in front of everybody (including the kitten plates)? And yea, their couplings seemed to last an infinity and a day, and the Goddesses and lesser Gods wept tears of pure joy at the sight of their purest of pure love. Thankfully, Pansy thought ahead and cast a "sperm-be-gone" spell on Draco, so that there would be no ickle Dramiones or Hermcos coming forth in nine moon cycle's time, and it was good because children are not sexy.*

And with the Marietta scenario... okay, so you're starting a secret club that the students need, yet, for psychotic teacher reasons, it is not allowed to exist. In fact, if it's ever discovered, expulsion and all sorts of completely crazy penalties are to be expected for everyone involved.
Several of the people involved, whether you like it or not, have parents in the very institution that is Cramping Your Style. What do you do?

I'm honestly surprised she didn't take more precautions with the DA, instead of just a (somewhat painful and humiliating) punishment for tattlers. It was cruel, yes. There are other things she should have/could have done. Completely out of line, OMG Hermione's like a Death Eater in the way she tortures her fellow students? No.

(For the record, she was also putting Harry and Ron at risk if either of them chose to tell Dumbledore/McGonagall/Snape, their "safe" teachers. Your name's on the paper and you break the number one rule of Fight Club Dumbledore's Army? You get pimples, my friend!)

And I don't think it's fair to say Marietta was in the right, either, since the DA was nothing more than a club for practising spells. It was against the rules, yes, but the rules it was against were recently introduced and completely ridiculous - and I do believe that we are allowed to judge if a rule is stupid or not, it's not always smart, but sometimes it's necessary.

In short, Hermione is not perfect, but stop pretending that Draco or Lucius or precious little Tom Riddle are. Because they're not morally reprehensible in the slightest, noooooo.

*I seriously messed that one up, there should be at least twenty mentions of "seed" and "bountiful busoms" and "maidenhood" in there.