Grimnir is a monster, literally. The Norse call him skraelingr. To the Irish, he is the fomoraig, and to the English he is an orcneas. Born and raised to do war, for and against the old gods. Immortal, they spend their endless lives, longing for glory in the final battle of Ragnarok.
So Grimnir’s disposition is already brutal, but to add to it, he’s the very last of his kind. To say he’s a pissed-off is a gross understatement.