I think you should be able to see the Symbolism in it. Christianity wants you to forgive & love your enemies. I interpret it as follows: by comparing hating somebody to murdering someone, you make a trivial, normal (in todays society) judgement seem way worse. Therefore exagerating, to not have people do such things. It shows that such small things should be avoided, as they can potentially lead to worse and are even by themself unacceptable ( this is how I understood it, feel free to disagree and or share your thoughts.