Because of stories like this one.
Regular folks don’t have the right, in the normal course of events, to take the law into their own hands and enforce their own justice. But regular folks do have the right to defend themselves and their families, fellows and property, against threats.
We were not there when the son in this story chased down one of the thugs who was attacking his mother. We don’t know how much force he used. Generally speaking one is not allowed to use force that is greater than what a reasonable person would have used in the same circumstances, and this always varies depending on the details of each case. We were not there, we didn’t see it happen, we do not know whether undue force was used in this particular case. But we know this: When the state prosecutes regular folks who were suddenly put in a very difficult situation where they had to react quickly to avoid or deal with a very dangerous threat harder than it prosecutes hardened criminals with a rap sheet this long for attacking and threatening innocent people, we no longer have a state that protects decency and order and we must – peacefully but firmly – work to change that.