I think he’d be really sad. But he would have seen it coming. He knew the Death Star was being built, he knew what it was for and he would dread every second of its existence. He would take every opportunity to undermine its construction in a way that couldn’t be traced to him. He would do everything in his power to curtail the danger to his people. And when Alderaan is destroyed, he may partially blame himself for not acting faster or more decisively. Despite it all, Thrawn values all life, even Human, even the people he will never know and the loss of an entire culture would be devastating to him, as well, because he was allied with those responsible.
Thrawn would mourn their loss but the destruction of Alderaan would also sharpen his focus. He would see to it personally that the Ascendancy was protected. He would make sure Alderaan never happened again.
(I personally headcanon that Thrawn did visit the Death Star before the Battle of Yavin where he met a certain scientist named Galen Erso… And then Thrawn never set foot on the Death Star ever again.)
Of course there is this holo:
Now in this headcanon fanfic, I wrote – http://archiveofourown.org/works/6466003/chapters/14799058
I was sent out of the known Imperial space by the time the Emperor commissioned a new Death Star.