I'm going to hope I'm not getting trolled here...
I consider myself libertarian, and I agree that the role of government includes criminal law and policing. Of course, you and I might disagree on what policing means, but that is an entirely different argument. Generally speaking, I think the easiest description of the role of government is "securing the rights of the people". However, like many quick throw-away phrases this really misses the depth and nuance that a real discussion of the role of government should have. It's intended as a starting point and as a way to help establish the basis of my beliefs with others. If you start digging into it, and I think this is common among libertarians, you will find a set of beliefs which go back to the Natural Rights theory of morality and a belief that civil rights come from those Natural Rights (Life, Liberty, and Property).
I have yet to figure out why non-libertarians seem to insist on equating libertarianism with anarchism, I guess it's just the stupidity of the anti-libertarian dogma. The two are not the same, though they do share many tenets and many people who are anarchists but don't want to be called out as such will claim to be libertarian.
Just to use the examples you cited:
To deter murder. - Without a government to imprison a murderer, people will take the law into their own hands by killing people they suspect of murder. Whether they are actually guilty or not. A process with soon escalates to feud. Libertarian utopia = everyone else's hell.
Absolutely this is the role of government. Part of the reason that written laws and formalized systems of criminal punishment were created was the fallout from blood feuds. A person has a right to life, if that is violated by another the violator should be punished. Without a formal system of government and criminal law, this would quickly turn into a mess (Hatfields and McCoys, anyone?). That said, if a person really wants to kill themselves in a way which does not harm others, well, it's their body. I agree that they may have mental issues which need resolving, and perhaps we should try to convince (not force) them to change their mind. This includes the use of drugs (be it nicotine, alcohol, or heroin) all of them are dumb, but it's your body to destroy if you want to.
As a less obvious example, you want the government to have no role in firearm control. So there's absolutely nothing to stop your neighbours giving guns to their children to play with as they feel like.
Quick question, what is stopping my neighbor from doing that now? "The Law" is not stopping it, it can't. If my neighbor was to get a sudden itch to hand my child a firearm, and neither I nor an adult with some sense was there to stop him, the laws of physics aren't going to suddenly stop the transfer because we have a law against it. You could try "The Police" as your answer; however, there are not enough resources to have a police officer standing over every child 24 hours a day 7 days a week. Quite simply, if my neighbor wanted to hand my child a firearm to play with, and I wasn't around to stop it, nothing would stop it. And this is where you and I will likely begin to digress in our view of the police. The police do not exists to stop crimes. They can't, there are simply not enough of them and probably never will be. The role of the police is to investigate crimes after they happen, arrest the person whom the evidence points to as the guilty party, and bring them to the justice system to be tried.
How are you going to feel when the kids next door are playing cowboys and indians with real guns? You may not care about them, but what about when the stray bullets come on your side of the fence?
Again, what's stopping this now? It's not the government, it's their parents. The government has a pretty poor track record of stopping this behavior. Take a look at places like Oakland or Los Angeles where the gangs have gotten out of hand. The government and all the laws you hold up as panacea are not stopping the problem. There has been a massive breakdown in parenting and simply teaching children to respect life. Now, The solution is likely to involve the government and a huge amount of social work and pressure, but just making gang activity illegal isn't going to stop it, it hasn't yet.
All this said, I'm ok with drunk driving and no texting while driving laws. People are creating a direct danger to the rights of others through through their own choices. I'd even go along with restrictions on billboards on the sides of highways, they are designed to distract drivers, which creates a direct danger to others. Seat belt laws and motorcycle helmet laws, not so much.