Responsibility to Retreat
The order of events here is not necessarily important. The fact is that they were not police officers, were threatening, and they were likely at this point on his property (needs clarification.) If not on his/her property (ie on public or another person's property) and retreating endangers him/her or leaves another person exposed to
2 Severe bodily harm
3 Sexual penetration
more than standing one's ground, then this authorizes use of lethal force. I do not see the severe bodily harm or sexual penetration test met in this case, and certainly not death.
If they were on his property, then those three tests do not have to be met*, since he no longer has a responsbility to retreat whereas the rent a cops did.
It is the so called "Stand Your Ground" test. It applies without restriction in almost all states within the four walls of the owner's house or car, meaning there is no responsibility to retreat. Outside the home, car, or place of work there are various levels of responsibility to retreat with risk of any of the three conditions removing any responsibility and authorizing lethal force.
In the end he should have called the police immediately, and asked the rent a cops to get off his property or be charged with trespassing. Then wait for the law enforcement officers to arrive to mediate.
Keep in mind if you kill someone you will be tried for murder which means pulling out the gun is a last resort to anything else. If the judge does not agree with your judgement you WILL go to jail for manslaughter or murder two. Police officers are held to the same standards. Anyone that kills another regardless of situation must be acquitted for the killing. Plus killing is something that haunts dreams, and you will have to live with it.
*Perhaps a Colorado reader can enlighten us on the specifics of the retreat test outside the four walls of the home or car in that state when using deadly force.