Are the deeds of Vibhishan in Ramayana justified?

Vibhishana was mentioned in Aranya Kanda by Surpanakha, the sister of Vibhishana, as Dharmatma - virtue-souled. She says Vibhishana was not only a virtue souled but also does not behave like a demon.

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”

In Yuddha Kanda it was mentioned that he had heated arguments with all the demons in the Ravana’s Assembly, when he advocates restoring Sita to Sri Rama.

Ravana gets angry with his attitude and scolds him and asks Vibhishana to get out of the Assembly. As Vibhishana could not stand to insult he leaves Lanka. While moving out he says :

स त्वम् भ्राता असि मे राजन् ब्रूहि माम् यद् यद् इच्चसि |
ज्येष्ठो मान्यः पितृसमो न च धर्मपथे स्थितः || ६-१६-१९
इदम् तु परुषम् वाक्यम् न क्षमामि अनृतम् तव |

"O, king! You are a brother to me. You utter whatever you like. Elder brother is like a father. Even if he is not staying in a right path, he is to be respected. But I cannot forbear these harsh words from you."

सुलभाः पुरुषा राजन् सततम् प्रिय वादिनः || ६-१६-२१
अप्रियस्य तु पथ्यस्य वक्ता श्रोता च दुर्लभः |

"O, king! Men who forever speak pleasing words are easy to be obtained. But one who speaks of useful but unpleasing words as also the one who listens to them, are rare to be obtained"

तनर्षयतु यच्चोक्तम् गुरुत्वाद्धितमिच्चता || ६-१६-२५
आत्मानम् सर्वथा रक्ष पुरीम् च इमाम् सराक्षसाम् |
स्वस्ति ते अस्तु गमिष्यामि सुखी भव मया विना || ६-१६-२६

"As an elder brother, bear the words spoken by me, who desire your welfare. Guard this city and yourself along with demons, by all means. Let all be well with you. I am departing. Be a happy person without me."

After that he takes refuge with Sri Rama, who anoints him to be the King of Lanka. Having joined Sri Rama’s camp, he fights for Sri Rama’s cause,i.e, for Dharma.

Hence, his actions are justified.