Well I mean, most of the time murder happens as a highly emotional short-term reactive action. People don't really think of the consequences, they are just angry or distressed enough that their mind tells them that this person has to be destroyed for their own satisfaction/relief/order. Society can stress the life-ending consequences of murder to the common man, and we do, but the fact of the matter is that people don't really have a sense of what the punishment REALLY entails until they are experiencing it. You never really understand anything until you experience it. They have an idea of it but it's not really truly real, it's never imagined in the day-to-day this-is-how-my-life-will-inescapably-progress-after-I-do-this future scenario. I guess a large part of why this is is that nobody ever really wants to think about it, so they don't put any more thought into it than they need to.
I would say that popular culture movies like the Shawshank Redemption and Oz and such help deter this by giving some idea of what it would be like to spend life in prison, but you usually don't have a good incentive to sit down and watch a prison movie unless it's about something other than how shitty prison is.
There's also just crazy or imbalanced people, who exist and there's probably a way to better catalog them, but that means that police and government oversight plays a more active role in our lives and this is not something a lot of people appreciate. Also, there's a lot of grey area in what "potentially dangerous" might be defined as to our police force.
Also, it's a matter of numbers. There's usually just more people than a police force could effectively monitor. If we're going to allow autonomy and freedom, than we're allowing the possibility that people will use this to hurt us.