Couple things here.
First off most jurisdictions have multiple categories of murder. This is not manslaughter, that is a different situation.
First degree murder or what is commonly called "cold blooded premeditated"murder. A murder with particularly evil intent or heinous process. Think son of sam, charles manson, dahmer and so on.
Second degree murder. This can be thought of like the scenario that started the thread, A heat of passion situation where "rational" thought and normal societal hindrences drop in a fit of rage.
Then you have manslaughter, a situation where the ending of life was NOT the intent but happened as a secondsary result of the act that ended up causing it. Think acidental discharge, killing with a motor vehicle in a traffic collision (non intentionally), giving a drug to a person which causes a death and so on. There are usually different levels of manslaughter as well like murder.
Secondly, in more than one state the jury CAN decide which is the appropriate charge for conviction. The state may prosecute for murder 1 for example and the jury can find for 2nd degree instead. A sentence hearing can be held immediately afterwards to determine the penalty. It all depends on how the law is written in the pertinant state.
In the scenario that started the thread I'd guess that 2nd degree would be tried and convicted. The penalty will also be variable dependant on the mens rhea (eveil intent) and how it was doen. If the suspect WAS the rapist and Dad tortured the guy to death then it's murder 1 likely or murder 2 with max penalty. My guess in this case 2nd degree murder with 15 to 20 years and time off for good behavior. Figure he'd be out in 7 years.