Yes The Parallax View was one well sick ending as it was pretty dour all the way though you were expecting the hero to win though at the end.
Race With the Devil had a downbeat ending as did Blow Out, Also Von Ryan’s Express, this ending was altered from the book at his request of Sinatra, because at the time because it was vogue in the films at the time for the hero to get killed at the end as in Madigan. Which reminds me of The Bedford Incident, Fail Safe. And although a lighthearted one Dr Srangelove
Do not forget all those horror anthologies both of the UK and US type they generally have bad endings in the stories and the rap round ones as well
But there was bad endings back in 1948 in Saraband for Dead Lovers PLOT SPOILER the hero is crowdedly stabbed in the back

whilst fighting a duel by the villains henchman standing on the sideline and the heroine is locked away.
The Omen were shock endings as in both cases you expected the stories to be tied up (well the first at least)and when it was in the third one, they went and opened it up again in a truly ghastly TV movie.
Some I really hated as well as The Parallax View were Fallen and The Astronauts Wife (Rosemay's Baby,?)as they to had the hero on the back foot all the way through yet they devise a really ingenious way to foil them then the villain gets out by a cheap method.
Stepford Wife’s, Arlington Road and OHMSS.