Commentaries:
This section shows that Jesus thought as we do regarding the time of the end. The word "fire" in verse 49 indicates "judgment." "I am come to send judgment upon the earth." And then He says, "What will I?", meaning, "How I wish that it was already kindled!" He wanted to get on with it, even as we do today. However, in verse 50 He shows that certain things have to come to pass and be done first. In this particular case, He is referring to His death and resurrection.
John W. Ritenbaugh
Where Is the Beast? (Part One)