I think what she means by "accident" is that death perhaps wasn't her intended outcome. There are a lot of pieces we, the public, are missing. Like, when did she actually take the suitcase over to the church.... Before taking Sandra, with Sandra, or after she was killed? That would point to premeditation of murder. And lots of other things we are unaware of.
One scenario that could have occurred is that she intended to drug and molest Sandra at the church... perhaps even photograph or videotape this for kiddie *advertiser censored*. Sandra could've been accidentally overdosed. Or, Sandra could've fought back and screamed she would tell... this was NOT in MH's fantasy of how this was gonna work. She could've then lost it... tried to shut her up, and smothered/strangled her. These are just a couple of reason a mentally disturbed person might think it was an "accident".
Then again, she has proven herself to be a pathological liar who loves the limelight and attention. VERY bizarre. Many people think she hung her own noose, so to speak. But I think that LE KNEW she was the perp early on, but couldn't prove it. I mean, I have never heard of round the clock police guards for someone who tries to commit suicide. Typically they just call social services and a psych consult. I think LE knew early on she was the perp, and gave her just enough rope to hang herself. Her behaviour became erratic.. what with the multiple press interviews, text mgs, etc. It was at that point that I believe she was "breaking".. LE knew it, called her in, and got a confession.
There is a rumor going around here in Tracy. I hesitate to state it, as it is RUMOR ONLY. But some are saying she was making a snuff film. Don't know if I buy this or not, but if so, I hope she FRIES!!