+Callum J Hackett
The crime of passion sits in a gray area, for me, and the instinctual reaction to protect one's reproductive opportunity is also hardwired. It would seem that some people couldn't control such a response, and were a jury to let them off the hook entirely, I might blink an eye, yes. However, if the incident were truly isolated, truly fueled by whatever ill-defined evolutionary drivers were at play, I think one couldn't consider incarceration rehabilitative for this person; you'd have to call it punishment, and punishment seems to me to be standing on increasingly infirm ground.
Sticking this person in a cell would markedly not
achieve two things: it would neither "fix" the murderer nor protect innocent people (not likely, anyway, though it might protect the legally innocent potential cheaters of his/her future relationships). I suppose you could make the argument that dangling the specter of incarceration for such acts might serve as a deterrent, but I'm not sure this possibility would often occur to someone who'd just walked in on a scene like that. To me, it depends on what we deem the purpose of any punitive action to be taken, the impulsiveness of the murder, and the ostensible risks and benefits with regard to rights protection, both to the accused as well as to the public.
However, it would seem odd to let the person off completely, without any kind of consequence for a passion-driven murder. I'm not sure jail would be the answer, though.
To answer your other question, I agree with you that the response here is largely emotive, which is both unsurprising and understandable. (I don't know how many times you're going to have to say you'd probably do the same thing. I don't have kids either, but the simple thought of something similar happening one of my siblings or parents sparks a palpable rage.) The fact that this girl was molested, of course, is utterly horrible, and positions this issue, as far as the father's impulse is concerned, into more defensible ground than the hypothetical you posed. But I think there is a rational basis that happens to support the emotions here, and justifies the court's outright forgiveness of the father's actions: the stakes, should the father have failed to protect his daughter, are much higher than those the betrayed lover would have faced. This was a defense of person rather than of dignity or pride or reproduction.
Further, the nightmare is one both the father and daughter will have to live with for the rest of their lives; they have both been violated irreversibly.