The concept that you call "intervening supervening criminal act" (I am not a lawyer and have no knowledge of this term) seems to require that a person use common sense and that a judge would dismiss with prejudice any lawyer who would dare to file such a frivilous lawsuit. However, we all know that there are activist judges out there who will allow such stupid lawsuits to go forward WITH FULL KNOWLEDGE that the primary reasons for this lawsuit are publicity and the hope that they can use lawyers fees to destroy the gun industry.
What is to stop that same judge from allowing a lawsuit against GM and Anhauser-Busch because their dealers sold booze and a car to a "known drunken driving offender" who then got drunk and plowed into a group of kindergarteners?
The problem is that this concept is a taught if first year law school but it is disregarded by judges when considering dismissal motions.
Before a case gets to the jury, the procedural motions regarding evidence and surviving dismissal motions and summary judgment motions. Some Judges have a very sincere consideration that a criminal act by a person should be a fact for the jury to decide. HOWEVER, I can VERY comfortably say these suits are not intended to reach the jury. They are intended to force a party to mediation. That ALONE should be grounds for dimissal. If a case is not brought with the intention of going to the jury, then the lawyer should be sanctioned.
The civil trial courts are not there to force negotiations they are there to make decisions of fact.