not only should we not try to prevent it, but it should even be easier.
It's already easy enough. You just can't make it easier. What you can do is to make it less painful. Is the fear of pain a deterrent? Perhaps, to some. But the car exhaust (CO) will kill you painlessly; some sleeping pills (barbiturates) will do the same. Heroin will do you in as sure as a bullet; and not only you won't suffer a pain, you will be rewarded with the final performance. It's far easier for most suiciders to just park their car in the garage, close the door, and let the CO kill them, than to look for a gun and then shoot themselves. It is very painful, by the way, and very messy - shooting yourself is not a good way to leave this world.
The society will not notice the outcome of their decision anyway; there are 6+ billion people on the planet already, it's not like we are endangered species or something. If someone wants to make room, it's their right. Not that I encourage them, of course. They are just free.
I know a guy who committed suicide and a girl who attempted suicide and no one is happy that he succeeded or that she failed
Romeo and Juliet, something like that? Those were successful all the way through. Does the society want them dead? Not really. But, darwinistically speaking, the society benefits from mentally stable people, not from head cases. Those *should* evolve out, in the grand scheme of things. Like taxes, if you support a certain behavior you get more of it. There are people who try to commit suicide repeatedly (and fail N-1 times out of that.) Then firemen are summoned, the police, and the doctors... what for? In the USA the Constitution guarantees your right for pursuit of happiness, but it does not define what form it may take. If you cannot live without your man|girl, don't. Will I be sad? Probably. But I cannot tell you to suffer for years, if not for the rest of your life, just because it is in my personal interests, either political or religious, to keep you alive. That would be awfully selfish of me. On that subject:
can't you at least acknowledge that more people killing themselves is a bad thing?
Bad thing... bad thing... bad to who? What metric are you using, and whose viewpoint? Per the blind and deaf quadriplegic, his life is over already. Per his brother, he must be kept alive until brother's own child can inherit his house. Per his wife, he should die immediately, so that she inherits. Per his aunt, Jesus the God personally told her that suicide is a sin, so the poor injured man must be kept alive for as long as possible - even though he suffers physically and mentally. Who is correct here, in this sea of incompatible interests? (This is a dramatization of a real world scenario that played out in Florida.)
So when you say "bad thing" you need to qualify this statement. The nature doesn't have bad things. Things can be declared good or bad only by an observer who has an opinion.
Actually I'm guessing the ones who don't get caught or killed stop on their own once they pass their mid-twenties.
I'm not sure where you live, but in most countries criminals cannot stop. There are the usual socioeconomic reasons for that. There is not enough jobs even for citizens who never jaywalked. What chance, in your opinion, a man with a burglary or a theft under his belt has? How many store managers will be happy to give him the keys to the money box? The only jobs that are left for them are menial jobs, like digging of ditches. Maybe one can become a licensed professional, like an electrician or a plumber, but that's not easy - there is a requirement for apprenticeship, and with that see above.
Can a criminal reform? Yes. Most of those success stories are from white collar crime, where for example an accountant made a "mistake" toward his own bank account. Just once in his whole life. He won't do that again. Kevin Mitnick is a good example. Some violent criminals embrace religion in prison and also become ex-criminals. The vast majority, however, is stuck in the vicious circle forever. They don't know how to live differently, and the society rejects them even if they try to end their wrong ways; they become career criminals.
With regard to "hardened killers", there is no shortage of those. Gang initiation rituals sometimes include killing of someone. There are obvious reasons for a gang leader to require that. Many homeowners are injured and killed during home invasion. Nobody would be concerned about an imaginary problem; the people are concerned because the problem is very real. It is exacerbated by the fact that most homes in the USA are open to anyone; you are separated from the street with just one flimsy glass door. Burglars throw a stone through that door (in the back of the house, usually,) and if nobody comes out to investigate then they come in and gather valuables. If you are unfortunate enough to catch them in the act, they may kill you. Many burglars are desperate druggies in search of money to buy another dose; they won't even consider your life sacred; they will kill you for $10 (that has also happened.)
I cannot say much about the distribution of violence among criminals. But my own understanding is that meek criminals do not exist. They simply cannot survive among their own kind. If they are lucky enough to get arrested early, they are most likely to mend their ways. The survivors, on the other hand, are someone to fear - they are graduates of a school that does not forgive a weakness.