Actually Don was supposed to shoot himself so the doctor could have replaced his heart with one he keeps in a jar. He impersonates the dead doctor (instead of running away) and then begins conducting experiments on patients before he goes mad himself...
Maniac has a reputation for being one of the worst films ever made. It is a fair assessment, though the film is just so weird and off the wall as to be highly watchable despite the ineptitude, bizarre plot and strangeness going on. It has some great lines, when asked if Don has seen some missing cats and if he is using them for experiments he says he thinks too much of Satan to do that!
