Time changes people sure. But events change them more. This is Abaddon after the failure of the Heresy. During the Heresy and even the Great Crusade, his faith and belief in Horus was absolute, completely confident in their path and victory. But then Horus, his beloved and idolised father is killed. Not just killed but obliterated and erased from existence. The war is lost, they're forced to flee before the wrath of the loyalists and erased from Imperial records, the great and famous/infamous Abaddon, known across the Imperium forgotten.
Then within the Eye the remains of his Legion are slowly hunted down and murdered by the other traitor legions, their once proud and famed reputation ruined and forgotten, only their 'cowardice' at the Seige remembered.
I'd say his change is justified.