Was Kumbhakarna justified in fighting alongside Ravana?

 

a) Kumbhakarna was mentioned in the Ramayana in the Aranya Kanda, when Surpanakha introduced herself to Shri Rama, as the sister of Ravana, Kumbhakarna and Vibhishana.

रावणो नाम मे भ्राता यदि ते श्रोत्रम् आगतः || ३-१७-२१
वीरो विश्रवसः पुत्रो यदि ते श्रोत्रम् आगतः |

"My brother is valorous and mighty Ravana, the king of demons and the son of Vishravasa, if ever you have heard of him.

प्रवृद्ध निद्रः च सदा कुंभकर्णो महाबलः || ३-१७-२२
विभीषणः तु धर्मात्मा न तु राक्षस चेष्टितः |
प्रख्यात वीर्यौ च रणे भ्रातरौ खर दूषणौ || ३-१७-२३

"And the mighty Kumbhakarna who will always be in profound sleep is my brother, and the virtue-souled Vibheeshana too is my brother, but he does not behave like a demon, and two more bothers of mine are Khara and Duushana who are renowned for their bravery in war.

b) Kumbhakarna made his appearance in the Royal Assembly hall of Ravana, when the news of Shri Rama along with his army reaching the other side of the ocean reaches Ravana. Ravana narrates as to how he abducted Sita, what Shri Hanuman did in Lanka, as Kumbhakarna was sleeping at that point of time.

Kumbhakarna admonishes Ravana, but later assures that he would annihilate Shri Rama and his army.

अयम् हि सुप्तः ष्ण्मासान् कुम्भकर्णो महाबलः |
सर्वशस्त्रभृतम् मुख्यः स इदानीम् समुत्थितः || ६-१२-११

"This most powerful Kumbhakarna, the chief among all the wielders of weapons, was indeed sleeping for the last six months and now, he woke up."

यदा तु रामस्य सलक्ष्मणस्य | प्रसह्य सीता खलु पा इहाऽहृता |
सकृत्समीक्षैव सुनिश्चितम् तदा | भजेत चित्तम् यमुनेव यामुनम् || ६-१२-२८

"You should have contemplated thoroughly that very moment when Seetha was brought here forcibly only after a single impulsive thought from the hermitage of Rama, who was accompanied by Lakshmana, even like the river Yamuna fills the depression at its source the moment it descends on earth."

न्यायेन राजकार्याणि यः करोति दशानन |
न स सम्तप्यते पश्चान्निश्चतार्थमतिर्नृपः || ६-१२-३०

"O, Ravana! A king who performs king's functions with a mind duly ascertained by justice, will not repent thereafter."

अहमुत्सादयिष्यामि शत्रूंस्तव निशाचर |
यदि शक्रविवस्वन्तौ यदि पावकमारुतौ || ६-१२-३६
तावहम् योधयिष्यामि कुबेरवरुणावपि |

"O, ranger of night! I shall destroy your enemies. I shall do battle with the two brothers, even if they are Indra and the sun-god or the god of fire and the wind-god or Kubera (the god of riches) and Varuna (the god of water).

गिरिमात्रशरीरस्य महापरिघयोधिनः || ६-१२-३७
नर्दतस्तीक्ष्णदम्ष्ट्रस्य बिभीयाद्वै पुरन्दरः |

"Entering the combat with a mountain-sized body and fiery teeth, roaring the while and attacking with an immense bludgeon, I shall strike terror in Indra himself!"

वधेव वै दाशरथेह् सुखावहम् |
जयम् तवाहर्तुमहम् तयिष्ये |
हत्वा च रामम् सह लक्ष्मणेन |
खादामि सर्वान् हरियूथमुख्यान् || ६-१२-३९

"I shall try to fetch victory, which brings pleasure to you, by the killing of Rama. By exterminating Rama along with Lakshmana, I shall devour all the leaders of monkey-troops."

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Here, Sage Valmiki presents the subtle difference between Kumbhakarna and Vibhishana, both were brothers to Ravana.

While both did not approve the action of Ravana in abducting Sita, Kumbhakarna preferred to remain with the line of action of Ravana, but Vibhishana advised repeatedly Ravana to restore Sita to Shri Rama.

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c) Next, he was actually woken up while he was in the next round of sleep for 6 months, after Ravana was defeated by Shri Rama in the 1st round of battle.

Kumbhakarna fights valiantly, as promised, and finally gets killed by Shri Rama.

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The character of Kumbhakarna represents the group of people, who though knew very well what is right and what is wrong, but still cannot take an independent and righteous decision.