Sinful acts in general are doing something that is not wrong in itself, but at the wrong time and place, for the wrong reasons. In I Timothy 4:4, 5
Paul says, “For everything God created is good, and nothing is to be rejected if it is received with thanksgiving, because it is consecrated by the word of God and prayer.”
The problem is that we often do things which we feel God forbids, but we do them anyway, so obviously we cannot do them with thanksgiving and prayer, and this makes them sinful. Romans 14:23b, “whatever is not from faith is sin,” was written about eating, which is obviously not sinful in itself, but the motive and circumstances can make it sinful. It wasn't wrong for David to marry Bathsheba. It's only wrong under circumstances and time that he married her. This is apparent in our everyday activities if we give genuine attention to it. Someone is going to say, oh man, stop that crap. So, raping get timing and circumstances? Killing get timing? Wait a minute! I'm not here talking about a type of sin, but sin in general. Why an action is a sin.. I'm basically saying you can do the same thing under different circumstances and time, and the results, one could be wrong while the other would be right. Raping for example, is a name given to a wrong done, and a wrong is an action done at the wrong time or circumstances. Raping, Adultery, Fornication is sex in a wrong situation. So, sex itself is not wrong. To kill is not wrong either.