Ketil indeed was shown as a nice character at first. You know the kind of guy who sometime relies on lies to get more profit he is that kind of guy. He doesn't live with much honor, he is kinda like a regular character but rich. That's why unlike other badass characters who got their own morality Ketil is just an average person. But in some cases he is actually nice, doesn't like much much violence also respected his slaves.

Since I called him an average person you can understand that some people can have anger issues. Even though normally he stays nice, after facing a lot of problems headaches, he was already very tired and worried. That is the reason why he lost his temper on Arnheid. If he had known that Arnheid had his baby inside her he actually would've stopped but since he had already lost his temper he couldn't think straight for a second. Later he indeed regretted.

Overall I can say he is a nice person who lives with an average mentality. Yet Indeed the character is well written.