Many comments to make here, First Nebraska has the Castle Doctrine, people need to lay off that. Castle Doctrine simply means you may defend yourself in your home without needing to retreat when facing a reasonable threat (in other words you invite someone in and attack them it does not apply). Since this did not happen (at least from what was reported it never entered the home) the Castle Doctrine does not apply.
So what you are wanting is "Stand Your Ground," that is what pertains to outside your home. That also would not apply from what is reported, Tyler Childs went to the home of his ex-girlfriend (angry, drunk whatever it does not matter how as long as he is doing nothing threatening) and is meet at the door by her father. Apparently when Childs knocked at the door or otherwise announced his presence, something completely legal. At this point Walter Campos produced a firearm, which would be a threat, escalating the situation and causing Childs to fear for his life and fighting to defend it.
If law enforcement felt that Childs was reasonably threatening Campos's life, well being, or that of someone else present; and Campos produced the firearm and used it in defense of his life or of another they could have let Campos go and then give the evidence to the County Prosecutor who could have agreed and wrote it off as self defense or could have reversed that and put out an arrest warrant.
Since the police arrested Campos it is apparent that even if they have doubts that they believe that Campos was the aggressor and thereby the shooting was not justified.
This has nothing to do with a duty to retreat which is what both Castle Doctrine and Stand Your Ground are about, this is asking to justify shooting someone because you think they need shooting. The Nebraska Supreme Court has held that any law saying that is unconstitutional, we had that law for a time back in the 1960's.