I am guessing it is because he unlawfully detained them and threatened them with the use of deadly force at gunpoint and with his dog, as well as assaulting one of them. Yes, they were tresspassing, but nothing in the article describes them as being a threat to him so his only action should have been to notify the boarder patrol or police and protect himself and family if necessary. As a former LEO he should have known the law and based upon what we know from the article and AZ Law (below) he did break the law thus violating their rights.
From The Arizona State Legislature
13-404. Justification; self-defense
A. Except as provided in subsection B of this section, a person is justified in threatening or using physical force against another when and to the extent a reasonable person would believe that physical force is immediately necessary to protect himself against the other's use or attempted use of unlawful physical force.
B. The threat or use of physical force against another is not justified:
1. In response to verbal provocation alone; or
2. To resist an arrest that the person knows or should know is being made by a peace officer or by a person acting in a peace officer's presence and at his direction, whether the arrest is lawful or unlawful, unless the physical force used by the peace officer exceeds that allowed by law; or
3. If the person provoked the other's use or attempted use of unlawful physical force, unless:
(a) The person withdraws from the encounter or clearly communicates to the other his intent to do so reasonably believing he cannot safely withdraw from the encounter; and
(b) The other nevertheless continues or attempts to use unlawful physical force against the person.
13-405. Justification; use of deadly physical force
A person is justified in threatening or using deadly physical force against another:
1. If such person would be justified in threatening or using physical force against the other under section 13-404, and
2. When and to the degree a reasonable person would believe that deadly physical force is immediately necessary to protect himself against the other's use or attempted use of unlawful deadly physical force.
Arizona Revised Statutes
Remember in America "All men are created equal" not all American citizens.