While there is no indication that he is tempered, his experience with Nidhogg has left a deeper mark on him than you seem to recognise here. There are a number of suggestions in various places that he has absorbed some aspects of Nidhogg's personality as a consequence of being merged with him, because he felt Nidhogg's emotions as his own during that time - but lucidly, it seems, and with the ability to decide what to do with those emotions.
He directly felt Nidhogg's rage and grief at Ratatoskr's death, and so he himself grieves for Ratatoskr at a personal intensity, and it is at least implied if not stated that he "thinks like a dragon" to some degree now - not because Nidhogg made him do it but because it was a natural outcome of being so deeply exposed to Nidhogg's soul.
I don't have references for in-game quotes, but the side story
From Azure Ashes covers the core of it.