There are various ways you can do this at home, but one of the best would be to go to a vet, either a normal one or an exotic animal one depending on the type of animal you're talking about, and killing them there.
If it's not a pet, and more of a rodent or something you're trying to get rid of due to it being a problem for your home / pets, then there are various humane methods online or at the store.
Mostly you just want to do it as quickly and painlessly as possible.
Interventions could be needed so the person the intervention is for can realize exactly how people they know see the situation of the person and so that the person can see exactly how their problem is affecting those around them. Counseling is also a valid option so the person can talk to a professional about their problem and seek out help to fix it.