I got that he was under a lot of stress and I expected Tom Cruise to be an arsehole for a significant proportion of the film. It was just too long for me. Several days later and he was still treating him like he wasn't human made me think that there must be something going on other than stress, the man must be evil because stress alone can't cause someone to treat another human like that for days on end.

For me, the part where I think he was irredeemably bad was during the Vegas sequence in the 3rd act where his brother wanders off and Tom Cruise's character still treats him like shit. It felt like even after he connected with his brother, he still treated him like shit.

I think it's also because I watched it 30 years after release and the kind of insults the character used might have been mildly acceptable at the time, but in my lifetime they have always been completely unacceptable. The kind of language that would get you fired and blacklisted from your job.