Re: Why?
We're all missing the relevant point. The relationship to the girl is irrelevant. It's irrelevant that the kid and the daughter had been having sex for a year.
The relevant facts:
1) The homeowner heard noises, thus having justified reason to believe his home had been broken into. He had no reason to suspect someone else had been invited in.
2) He armed himself and went to investigate. He found an unidentified naked man standing in his daughter's bedroom.
3) He had no reason to suspect there was anything consensual; the first and logical assumption is that an intruder had removed his clothes and was about to rape his daughter, so immediate defensive action was warranted.
We do not know if he could determine if the naked man was armed or had any offensive capability.
Thus, I would think the inherent right to defend one's home would preempt any assault charges. However, it does not appear that justice is being served here.
One other consideration: Not sure what the state laws are where this happened, but in Michigan the age of consent is 16. So if the daughter is under 16, she is considered incapable of giving consent, and any sex under that age is considered statutory rape. There are many, many cases of BF-GF having sex, in some cases their ages are only a few months apart. But one is over age, the other underage. So there are many kids convicted of unlawful sexual conduct and permanently required to register as a sex offender just because of loving sex with their BF/GF. Just? Probably not, but that's the law. In this case, if the girl was under age, it doesn't matter if she wanted her BF to have sex with her or not.
There was a situation reported last week in the local Kalamazoo Gazette. In the middle of the night, an upper-middle-age couple heard someone going through their car(s). The car(s) are in their driveway behind their home. The man gets his handgun, for which he is properly licensed (Michigan is a shall-issue state) and confronts the intruder. The wife calls 911.
There was a struggle, and the homeowner shoots the intruder in the leg, temporarily incapacitating him. Unfortunately, as a direct result of the struggle, the homeowner suffers a heart attack and dies.
So what happened?
In some jurisdictions, the homeowner and wife would have been charged with assault. But in this case, it seems justice is being served. The intruder has been charged with manslaughter in the commission of a crime. This just happened, so I'm sure it'll be months before the case is finalized.