Not all the time the law works like that. People get off scott free even when theres evidence in plain view that they did indeed do the crime.
You're not understanding. That's a different issue. That's a commentary on the judicial system, juries, etc. That has nothing to do with the actual law.
The law does not mandate that the authorities must establish that there is no evidence of a crime, in order to decide not to charge someone. That's what the original poster was in essence, stating. Once again, that is not how the law works.
Instead, they need actual evidence of foul play in order to charge someone.
You can't prove a negative. Probable cause to arrest a person means that the authorities have evidence that a crime was committed. The authorities are not required to determine there is no evidence of foul play. They have told determine if there IS.
That's how it works. That is the law.
Here is an example. Let's say two friends go hunting in the woods with one of their kids. One gets shot and killed. The other calls for help.
The surviving hunter appears suitably upset. Freaks out. He's crying. Upset. Explains that the two separated to look for deer and he thought he spotted one. Shot it but when he approached he realized it was his friend.
The authorities' job is to assess the scene. Judge the shooter's demeanor and credibility. Ask the child what he or she saw. Look at the distance and the brush cover and what the dead man was wearing.
If they find evidence that the survivor lied about where he was and his visibility when he shot the friend, if they find that actually, the two were arguing and the victim was shot at close range, then they likely have probable cause to arrest the shooter and/or for search warrants, etc.
But they don't have to search for NO evidence. They simply examine the scene and if there is evidence of foul play, then they will have probable cause to get search warrants to conduct a forensic exam of the shooter's computer, to see if he had anything to suggest he was murdered, to analyze his cell phone records or social media to see if he said or did anything to indicate hatred or plans to harm his hunting buddy, etc.
Again, when determining whether to charge a person, the law mandated they find evidence of foul play, not the other way around, and prove a negative.
Regardless, there is zero to suggest that the mother shot her child or that the authorities failed to do a comprehensive investigation. As I have proven through numerous links, four years old can and have shot people with handguns on numerous occasions. And the four year old was old enough to be able to tell what happened.