On the second scenario, you had time to think of the consequences of your act. You had time to think of the fact that this guy, douche as he may be, also has a mother, also has people who love him, and that killing him won't undo the offense that he and your hypothetical wife did to you. You'll still be just as heartbroken, and now you also killed someone. If after thinking everything through you still decide to go ahead and butcher him, you're a pretty bad guy that deserves a harsher punishment than on scenario 01.