Nice answers but NO. This is not the first mainstream film in which the murderer isn't punished by the law.
In Blackmail (1929), directed by Hitchcock, the lead gets away. She isn't arrested, but I believe she was forever hunted by the crime. Yet I believe she did it on self defense.
There is a rather heavy-handed line of dialogue late in this film in which one cop says to another that E.G. Robinson's character keeps telling everyone, "he should be tried and executed!" I feel reasonably certain this line was included to really hit the audience over the head with the idea that the personal hell Robinson has created for himself is the moral equivalent of being tried and executed. I think without that one line of dialogue, the Hays Code probably wouldn't have approved the film.