The police who did my (IC SN) welfare check and who (thankfully) left me with it- said I could take it in to them if I changed my mind. I'm not sure if that's wise- if you're not on their radar though. But then, it's more suggestive of someone trying to prevent their own suicide I suppose so, maybe it would be ok. They may start asking questions where you bought it though perhaps- if they are running investigations on it.
I've seen people say they just chucked it away or, poured it down the sink- which I also find disturbing- like other members. Seeing as how I'm sure it's very toxic.
Maybe irresponsibly, I'm willing to leave it (if I choose not to use it,) plus other things (we all no doubt have a bunch of stuff that needs proper disposal- paint thinners, dead batteries, broken electrical goods) for a house clearance company to dispose of. I'll make a note that a bunch of stuff needs disposing of responsibly.
I imagine the police may take it as part of the investigation too- if it's at the scene of suicide. I'll just put up warnings to wear gloves and dispose of stuff carefully.
Pretty disgusting but, I've wondered what they do with any vomit etc. I guess it goes into biological waste to be incinerated.