Ravan was fully confident that Shri Ram will not be able to even find Lanka let alone defeat him then one day a Vaanar comes and kills his youngest son. Throughout the war he had to see his kingdom gets destroyed and sons die. 

In Sarga 59 of Yuddha kand, he is defeated and humiliated by Shri Ram and spared. This is before Indrajeet and Kumbakaran even came on battlefield. If Rama had killed him swiftly then he would have spared lot of emotional pain of death of his sons and brother. At the end, he is left all alone before final Ram-Ravan yuddh.