If so, then:

• Presumably what's happening is some ritual involving the Deathly Hallows, carried out by Harry or Voldemort or both
• Most likely with the goal of defeating Death (somehow)
• So I guess the litres of blood are a requirement of the ritual (note: in view of earlier stuff about how modest the things sacrificed in rituals are, it had better be something as grand as defeating Death)
• Harry doesn't seem like the type to spill other people's blood even for such a goal (though, I dunno, he might) and Voldemort may quite possibly wish to spill Harry's
• So maybe it's Harry's blood, and he's (directly or, or letting Voldemort do it) sacrificing his own life to put a permanent end to death for everyone else
• Which would make a certain amount of dramatic sense
• In which case, probably the word being screamed is "No!" or "Harry!" or something of the kind
• Though maybe it ends up with him getting resurrected too; see, e.g., canon Harry Potter, Narnia, Jesus -- the first two being deliberately derived from the latter; not so congenial an idea to Eliezer as to Rowling (who is a Christian), but note Eliezer's "Ta-da" remark when Leah mentioned "Christus Victor" and divide-by-zero errors
• So maybe e.g. Harry needs to be killed while invisible to Death because of the Cloak of Invisibility; perhaps that sends his soul-or-similar-thing to wherever wizards' souls go, but without him actually being properly dead, and enables him to fix things up there (a very "Christus Victor" idea; maybe too much so)
• None of this seems to connect with the prophecy about "the one with power to vanquish the Dark Lord", but maybe as others have speculated the Dark Lord in question is Death or something

All, of course, pure wild ass-speculation.