For instance, a gun, a rope, or the like. What's the common way of referring to them? Assuming the suicide attempt of the person who used them was successful?
Example sentence:
We kept grandpa's shotgun in the garage. We never imagined that it'd become Tommy's [...] one day.
Means is very commonly used as to the method of suicide, in such cases as studying whether there is a link between availability of means and likelihood of attempts. (Last I read there was; potential suicides have a set of acceptable means and removing a means will lead to a drop in attempts because, but I'm not current on that at all).
Generally it refers to the general approach (e.g. "hanging") but it can also apply to more specific details.
This doesn't fit the one-word requirement of your question well. Your example sentence would have to be rephrased to something like "…it'd become the means of Tommy's suicide one day."
(While it would be reasonable to consider weapon, as such objects are literally such, that choice would probably be particularly poor because "suicide weapon" is often used to mean a weapon used to kill others in a suicide attack, like a kaiten or a bomb vest).
Instrument - the instrument of his/her demise. Or perhaps in certain scenarios, 'the method'.
Euphemism might be a nice tool in this case.
'Escape route,' or 'exit strategy' have that sort of feeling to them. They're also slightly less unfeeling than the literal statement 'instrument/means of suicide.'
As always, the context will dictate the appropriate word/phrase.
