Typically under the law a justifiable homicide occurs when a person takes lethal action to stop an imminent theat. The aggressor killed must have the intent and ability to cause immediate injury or death. If this father had discovered this man molesting his daughter and killed him at that time, he would probably be tried for manslaughter and acquitted as a justifiable homicide.
What actually happened is that the father discovered some type of evidence causing him to think that this man molested his daughter. He went to the man's home, broke in, and killed him with a knife. (The question of whether he found the knife there or brought it could be vital to the case.) Here there was no imminent threat. Instead, he sought out the deceased, went into his home, and killed him there, possibly with a weapon that he brought along with him. Right now it looks like intent, therefore first degree murder. Because there was provocation he will probably be able to get it knocked down to second degree murder or manslaughter, most likely manslaughter. However, because there was apparent intent to do some type of harm (he broke into the man's house, possibly armed with a deadly weapon) and there was no imminent danger, in my opinion it is not justifiable homicide. The jury might disagree and acquit him, I suppose it depends on the exact details of the case, which are not known.
OK, so technically it may not be 'Justifiable Homocide', but I'll bet you agree it's a justifiable homocide, right? (Obviously, assuming the dad was correct and the dead guy did it)
Bork is intersting on Jury nullification.
Helpful analysis, thanks