i mean, when your having to fight off ur inner daemons , starving off the red thirst and black rage, and having to deal with constant wars everyday in a galaxy trying to super murder you, i'd be pissed off too if some random battle brothers from different chapters comes along on my ship and bring along there bullshit too.
I suspect this is actually pretty accurate and the kindness they present towards normal humans is either a facade, an effort to help stave off their gene-flaws, or a form of nobless oblige.
To be honest he probably saw himself as a scientist and wanted to stay that way it was just out of necessity that he became the emperor because there werent any other people as qualifed as him.
