This thread is the first one that's seriously challenged my ethics! I'm all for people taking responsbility for their own suicide and not involving others. I'm a strong proponent of having my boundaries recognized and striving to recognize others' boundaries. I feel a lot of guilt for even small infractions of my ethics.
Relationships are an important consideration in suicide. Someone always has to find the body, and it's less traumatic if that person isn't personally known to them or anyone they know. I personally think it's awful to use the gun of someone one knows, and it's going to have a much greater impact on that person than it would on a total stranger.
In the valet scenario I think, okay, what if they had a rope in the car? That feels kind of shitty to me because a rope is not a weapon. It has tons of uses, very few of them violent. One would not reasonably expect that if they left a rope in the car, someone would see it as an opportunity to do harm to others or themselves.
A gun has one purpose: violence. Its uses are to defend or to assault, to injure or to kill. It could also be used for a paperweight, but it's not expected.
The OP has said that several people a week left guns out in the open in their cars. I acknowledge that for some this could be accidental. They may return to the car and think, "Holy fuck! That valet could have used my gun to rob a store and I would be implicated! Whew! Lesson learned!"
With regard to ethics, if someone is suicidal, taking a job for the opportunity to get access to a gun that's left out is targeting another. It's not taking personal responsibility for their own suicide, it's involving an unwilling other, same as the train method. In my personal ethics, that's not okay.
However, I'm not so idealistic and living in a dream world that I don't recognize people are going to do things that cause harm to others; that's the point of having a personal ethic and boundaries, so that we won't cause harm. But still, every single human being experiences suffering at the hands of other humans. Sometimes it's excusable to some degree, sometimes not.
If I heard about someone I loved having left out their gun in their car, and the valet used it for suicide, I'd see their personal responsbility in that, same as if they left their house unlocked and had things stolen. I'd be hurt for them, and at first feel righteous indignation on their behalf, but I also recognize that everyone -- even people I love, even me -- is responsible for self-protection. No one is ever at fault for being mugged, but if they walk down a dark alley alone in a bad neighborhood, they're not taking responsbility for their self-protection, either. I'm not going to point a finger and say, "You asked for it," but I am going to feel the discomfort of knowing they played a role in their own harm. Heck, I was sexually assaulted decades ago by someone who threw up huge red flags; he's at fault for assaulting me, but I did not take the responsibility to act to protect myself, and that's an uncomfortable fact. In all of these situations, I feel great compassion for the victim who suffered the consequences of making an error in either awareness or judgment; in my case, it was a combination of both. Humans make such errors, that's why I don't say, "You asked for it."
CONCLUSION:
The person who leaves a gun out in the open in their car, and hands someone else the keys to the car, is not taking personal responsbility for their property, their car and their gun, the second of which happens to be a tool whose sole purpose is to do violence. If they made a mistake by leaving out the gun, I have tons of compassion for their experiencing horrible unforseen consequences of that action, but it's a tool of extreme violence, and so requires extreme personal responsibility and caution.