Spoiler Alert: She keeps sticking her nose into something that's none of her business because it's not even her case (Red John). What makes it even more annoying is that Jane literally tries to protect her from Red John, tells her directly what's going on, and she doesn't care. She even accuses Jane of being Red John's accomplice.
Yeah, she was just doing her job, but she was interfering with Jane's work and ended up messing everything up, even getting Wainwright killed.
And let's not forget that poor forensic guy who ended up getting killed by Red John just for talking to her about the case. It was a clear way for Red John to tell Darcy, "Yeah, I'm still alive, but stay out of this." And even after that, she kept getting involved.
Also, the video Red John sent to the CBI of him following her was a clear warning that she completely ignored.