All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
We are not aware of any text from the Book of Allah or the Sunnah of the Prophet which indicates a special merit for the murderer of someone who insulted the Prophet to enter Paradise.
Moreover, the texts which indicate a threat of being deprived of entering Paradise, or the promise to enter it upon performing a certain deed or upon leaving a given prohibition does not mean that everyone who does that action is threatened by that threat, or that he will get the promise that is promised; rather, that is restricted by what is mentioned in other texts of fulfilling the conditions and the absence of impediments.
Ibn Taymiyyah said as in Mukhtasar al-Fatawa al-Misriyyah:
“Every Hadeeth that states that a believer will enter Hellfire or that he will not enter Paradise, then this has been interpreted by the Quran and the Sunnah that this is conditional upon the absence of these impediments. Likewise, the “promise texts” [i.e. the texts that give promise to someone to enter Paradise] are conditional on the absence of reasons that prevent one from entering Paradise, the greatest of which is that he dies as a disbeliever.
Allah knows best.