why did hanumAna not kill rAvANa, since he had encountered that demon before? (In fact, in Thai traditions, rAvaNa is actually slain by hanumAna)
mahAbhArata provides a convincing answer.
The second pAnDava, bhImasena, posed this interesting question to anjaneya his spiritual brother, when bhIma met him on his course to fetch a celestial flower for draupadi.
...Witnessing that dreadful and wonderful body of Hanuman, like unto the Vindhya mountain, the son of the wind-god became bewildered. Then with his down standing erect, the noble-minded Bhima, joining his hands, replied unto Hanuman saying, 'O lord, by me have been beheld the vast dimensions of thy body. Do thou (now), O highly powerful one, decrease thyself by thy own power. Surely I cannot look at thee, like unto the sun risen, and of immeasurable (power), and irrepressible, and resembling the mountain Mainaka.
O hero, to-day this wonder of my heart is very great, that thou remaining by his side, Rama should have encountered Ravana personally. Depending on the strength of thy arms, thou wert capable of instantly destroying Lanka, with its warriors, and horses, elephants and chariots. Surely, O son of the wind-god, there is nothing that is incapable of being achieved by thee; and in fight, Ravana together with his followers was no match for thee single-handed."
Thus addressed by Bhima, Hanuman, the chief of vaanara-s, answered in affectionate words uttered in solemn accents. "O mighty-armed one, O Bharata, it is even as thou sayest. O Bhimasena, that worst of Rakshasas was no match for me. But if I had slain Ravana--that thorn of the worlds--the glory of Raghu's son would have been obscured;--and for this it is that I left him alone. By slaying that lord of the Rakshasas together with his followers, and bringing back Sita unto his own city, that hero hath established his fame among men"....
Rama Setu and Sri Krishna - IV: Hanuman Tells Bhimasena About Setu