For the record I'm not a conspiracy theorist, and believe Ruby shot Oswald entirely of his own volition, but it's clear to me the Dallas Police seriously failed in their duty to protect Oswald from a public they knew was pretty much baying for his blood. I found this definition of what needs to be proven for a wrongful death/negligence tort in the US:
In order to hold the defendant liable in a wrongful death claim, the plaintiffs in the claim (usually through the estate of the deceased victim) must meet the same burden of proof that the victim would have had to meet had the victim lived. So, using negligence as an example, this means showing that the defendant owed the victim a duty of care, that the defendant breached this duty, that the breach of duty was a direct and proximate cause of the death, and that the death caused the damages that the plaintiff is trying to recover.
Now, as far as I can tell Oswald's wife and family never pursued any legal remedy here. Anyone know why?
And just to reiterate, I'm of the Vincent Bugliosi school of thought on the assassination (ie, the Warren Commission was pretty much correct in its conclusions). I'm not here asking this to prop up some idiotic conspiracy theory, but haven't been able to find anything that answers this question.
It is very difficult to sue a law enforcement officer for breaching their duty in the form of not doing their job properly. In the United States, law enforcement officers enjoy qualified immunity from civil suit. As the Supreme Court explained in in Harlow v. Fitzgerald: Government officials performing discretionary functions generally are shielded from liability for civil damages insofar as their conduct does not violate clearly established statutory or constitutional rights of which a reasonable person would have known.
The reasoning is that constant suit would interfere with the police's ability to function. Imagine if somebody brought suit every time the police failed to help them; this might create a breach of the duty of care every time someone is mugged, every time a home is burgled, and so on. The litigation would be endless.
To circumvent the protection that individual police officers enjoy, litigants sometimes try to sue the municipality itself or the police department. There are still immunity issues here, but if a litigant (for example) argues that the police department/city failed to properly train its officers, there is a chance they can win. Here is an article on "failure to train" as a basis for liability in s.1983 suits: http://corporate.findlaw.com/litigation-disputes/failure-to-train-as-a-theory-of-section-1983-liability-in-the.html
The downside is that it's virtually impossible to sue a law enforcement officer for not doing their job very well.
I could go on, but this article will explain it much more cogently than I could: https://leb.fbi.gov/2012/september/qualified-immunity-how-it-protects-law-enforcement-officers