the asphyx (1973)

aka Spirit of the Dead. The Horror of Death.
Sir Hugo (Robert Stephens) is a Victorian amateur scientist, who in experimenting with camera's manages to photograph the asphyx, a creature that escorts your soul at the moment of your death. Like all sane people he then comes to the conclusion that if he could trap it, it would make that person immortal. Hugo conducts a series of experiments , finally managing to capture his asphyx. But tragedy strikes when he then tries to trap his daughters (Jane Lapotaire), killing her. Paul (Robert Powell), her fiance, and Sir Hugo's assistant, then commits suicide by blowing up himself and the lab. Hugo now realising what he has done burns the letter containing the combination to the safe where his asphyx in contained. He has damned himself to live forever in torment. The film closes with a wiserned old man walking into oncoming traffic in a pitiful and doomed attempt to commit suicide.
An interesting concept, done competently. Unfortunately it demands you suspend your disbelief rather too often. I mean, if you were going to pretend to kill someone, why would you use a working guillotine, you are asking for trouble! But if you can do so it's worth it for the truly tragic ending.