I don't really understand the importance some folks are ascribing to the deer, and I actually don't think it's a "secret" in a sense that's significant to the narrative. What I mean by that is, what does the deer tell us, what could a revelation about the deer to Tara or the Scoobies tell them?
A: Willow has in general done some yucky things and tried to hide it from her friends.
Well, we and they already know that by now. At least two entire episodes this season have been devoted to the idea that Willow has done some yucky things in secret that her friends would be appalled or at least somewhat disturbed by. I'm not sure how much dramatic mileage Joss and co could or would bother to try to get from:
"Hey, in addition to lying to you about the yuckiness of the resurrection spell, hiding my increasing magic addiction, altering your minds, and endangering Damn's life, I also killed a deer!"
"Dear lord, when?"
"Well, it was part of the resurrection spell."
"Oh. Well, yeah, that spell sucked all around."
B: In specific, the resurrection spell was much more dark and hardcore than Willow let on to her friends.
"Hey, in addition to the horrendous gashes opening up on my body, and barfing up a snake, and that it had the effect of yanking her out of Heaven, you know what else was bad about the spell that I never told you? It required me to kill a deer!"
"Oh. That's where the blood came from then? Ew. Nice one Will. Remind me to get cake recipies from you like, never."
or
"Sweetie, it's been a huge struggle to let go my hurt and feelings of violation...but I can't deny I still love you...I guess I'm saying I'm willing to try again. Assuming you never pull that shit on me or the others or yourself again."
"Oh god, Tara, I won't mess it up this time, I promise, I love you so much....um...look...before you take me back..I guess it would be only fair to let you know that when i did the spell I didn't buy the blood..I killed a deer to get it."
"Um, bonjour, giving you the second chance here. Less last minute guilty confessing, more thorough "making it up to me" in the traditional post-flamey candle manner."
I just don't see it being a big deal, given what they already know. It's yucky, but they've been hip deep in yucky all season, this is is just a detail, they knew the blood came from something. And none of the scoobies, not even Tara, has indicated onscreen that what they want from Willow is a confessional accounting of her recent misdeeds. Some people who've had thier trust betrayed want exactly that, to be told every detail, to know exactly everything that's been hidden from them. And some recovery programs require it as well, that the addict or offender confront thier acts publicly. But that doesn't seem to be the stance of Tara or any of the other Scoobies. The misdeeds and thier severity are a given, it's the behavior going forward that they're concerned about.
*None of the preceding sample dialogue written by Joss Whedon, Mutant Enemy staff, or anyone with an ear for character voice. Member FDIC. Consult your doctor.
-len