I wonder what sets it off then ... In our case, it was finding the first crystal (I think). Or a similar 'crucial' event if you played 1.x. What did Ysayle experience that awakened the Echo? I mean, I know we don't know and therefore just have to speculate but - as you've probably gathered - I do love to speculate.
Anyway, about Dravanian attitudes to men and war ... The old ones, like Midgardsormr and Nidhogg, are very old, and most of their experiences with men have led them to conclude that conflict is men's natural bent.
In the scene with Tiamat at the end of HW Midgardsormr says ‘So, thy journey hath led thee to this accursed place. Canst thou hear it, mortal? The ceaseless keening of my kindred, steeped in five thousand years of suffering... Long before the Ishgardians and their base betrayal, there lived still more wicked men whose ambitions knew no bounds. Fearful of our might and covetous of our power, they devised a means to enslave dragonkind... Their vile misdeeds did set in motion a train of events whose repercussions are felt to this day. Beyond, thou wilt find one of my first brood. Hark thou unto her words. Partake thou of her pain. Think thou upon the suffering that the conflict betwixt our kinds hath begot.’ (Midgardsormr, Fetters of Lament, Heavensward).
And Tiamat claims that she and Bahamut and their many offspring lived peacefully in Meracydia for a long time. Until the Allage arrived. I know that Tiamat is bound to present things from her own perspective, but it seems to me that the experiences of Midgardsormr and the First Brood almost inevitably lead them to think men can't be trusted.
When Midgardsormr first meets us he calls us frail and ignoble; that's his experience of 'men' talking. It's only after witnessing our struggles and our resolve that he learns differently, at least of the WoL, and can say '‘Strong art thou, mortal─stronger than any other of thy kind. Having looked upon thy deeds, I am convinced. Thou art worthy to bear Her Light. The covenant bound me to thee, but 'twould seem our fates were mingled from the first. Though I will not fight thy battles, I will yet lend thee my wings. Come, mortal – we go to cast out the Darkness!’
So I insist on believing there is good in the Dravanians - some of them, at least! Nidhogg is clearly crazy and probably incapable of changing his perspective.