This film. which is also being shown on April 7th, is part of the cult strand season at BFI Southbank. You can find the full details here.
This dark, brooding low-budget effort opens in Vietnam, where young infantryman Andy Brooks (Richard Backus) is struck down by a sniper's bullet. At the same time in Andy's hometown, his poor mother is uttering a desperate prayer for Andy to come home... and shortly thereafter, he does. Despite Mrs. Brooks' exultation at her son's safe return, it becomes apparent to the rest of the family that there's something terribly wrong with Andy; he won't do much more than sit in a chair, staring blankly at the walls of his room... that is, until nightfall, when he prowls the town in search of human blood, which he extracts from his victims through a syringe and injects into his own veins. The first horror effort from director Bob Clark, who followed with Children Shouldn't Play with Dead Things! and the effective thriller Black Christmas, this haunting film (released as The Veteran in 1972) functions as a Vietnam-era variant on the classic story of "The Monkey's Paw" and was one of the first films of the genre to address the stateside reactions to the horrors of that war.
Here (and above) is the trailer.