Yep - that's the whole point of the tranquilizer dart. And seriously, that single decision is the main factor in why there's no forensic evidence at the house.
I do think all of you who suggested that Barry tried all week to get Suzanne alone out on a trail is a related strategy for him. He knew not to have any evidence of death, spittle, etc. at Puma Path.
He could even have gone for help after she "fell" from a steep trail (even better if she fell while taking a photo - it's the most common reason for women to die from falls in natural areas). But the strategy he chose has worked out pretty well for him, all things considered.
Because he had them already? Why go be seen buying syringes when you already have a device that works exactly like one? Plunging it into her back would have been a poor idea (it should be an intramuscular injection, and chances of hitting a bone on the back are pretty high). Leg or butt would be better.
He's a cheapskate and uses what he knows and what's on hand. He had boxes of those darts. I think he preferred hunting (using a gun) very very much.
