Restricting the sale of goods to minors is an overstep, but speed limits on a PUBLIC road are not. Most of the speed limits could, and should, be raised, but they do serve a purpose.
Of course you have to be justified in shooting someone else. Now, that justification depends on the culture that you live in. I have no problem with home invaders being shot, and I tend to side with the property owner on what force can be used with trespassers, or thieves.
It is not the government's duty to protect me from myself, but rather to protect me from others.
To be fair, "to serve and protect" applies even to protect you from yourself. Even beyond police, hospitals, your family and friends, etc. have some duty IMO to protect people from harm.
We're getting way off topic here