It’s an intentional decision on his part to write the characters like that. It’s part of what gives you a picture of these characters. If you look up an old Russian version of the movie you’ll see just how unhinged they appear in the Russian as well.

Dostoevsky himself had a crazy life, ordered to be executed with a last minute reprieve, suffered epilepsy, depression, and paranoia. He wasn’t going for realism but more in conversation with other writers of the time and probably his own experience of life.