Insulin is a pretty good idea; especially if your victim is already in the hospital with a "mysterious" disease. Using belladonna in very small doses [as in, you have to know what you're doing] can cause weird symptoms which can alarm ER doctors. Putting it through the IV can be a dead giveaway once the nurse checks the contents of the bag post-death. Normally, diabetics inject insulin in their stomachs or thighs, but that can bruise very, very easily. So injecting between the toes is your best bet. No bruising, and usually if the patient is already in the hospital for weird symptoms upon which there can not be found a diagnosis, COD is not searched for, but is put down as "natural death." Ironically, the killer can just through the murder weapon in the "sharps box," where used needles go anyway. The containers are opaquely red, and are considered haz-mat, so no-one's going to look through there to check for any extra syringes. The insulin container can go in there too; the glass bottles are small enough to fit in. Obtaining insulin is more than easy - just take a pet to a vets, tell them you're switching vets and that Fluffy needs few doses to get them through the week before the prescription changes hands.
Although I also "like" the idea of being exsanguination by icicle, especially since icicles are known to fall when they get too heavy. The murder weapons melts away, leaving no tells of any murder at all.