Tanith knew it.
The moment Sanguine got distracted from having weakened the Black Cleaver, she knew it would use this opportunity to strike.
A few years ago, she propably wouldn't have cared at all and maybe she would have even been happy. However now after the incident with the remnant and what Valkyrie had told her, she cared and she wanted to warn him.
Yet, it was clear, she wouldn't make it in time. Which is why, she pulled the last bit of strength the cleaver had left her, together and used her agility to rush to his side. And just before he would have sliced his scythe through Sanguine's throat, Tanith shoved the eyeless Killer out of the way.
Having at cause for her to get sliced instead of him and to have him having to watch her collapse to the ground.
At this sight a shudder went down Sanguines Spine and he cried out...
" N-NO" ...