Angrier and meaner. Is this normal?

Hi. First time poster. My dad was diagnosed with stage IV small cell lung cancer in August. He has mets to the brain, liver, and spine. He underwent brain radiation in addition to a few months of chemo. Repeat PET showed no improvement in the lung masses but the brain tumors did get better. Oncologist recommended no more chemo. My dad is...different. I understand that having a terminal diagnosis can make you depressed and can change your attitude. But he is so quick to anger now. He curses a lot. Belittles others. Yells about trivial things. My mom is at wits end. He forgets things and lashes out when we tell him stuff for a third or fourth time. We've been dealing with this for months. He doesn't want to go to therapy to vent his frustrations...doesn't think he needs it. I've tried talking to him about his mood but rarely do I get anywhere. We've been nothing but compassionate and sympathetic. But, like, having cancer doesn't give you the right to be an asshole. My mom has to apologize to anyone who visits and she's getting close to asking people to not visit at all. Has anyone else experienced this? Can this be a side effect of chemo and/or radiation? Or do some people just get ornery when they're at this stage? I don't want his last months to be mired in anger and frustration. Thanks for reading.