The adjective used to describe the emotion adopted by Marjorie when turned to face Warren is "Serpentlike".
Serpents are known for being animals with cold-blood, and although this is literal term to refer to their physiology, this word can also be attributed to humans in a metaphorical manner.
"Cold-blood" in this case describes an action being done without emotion or regret (just as animals, like snakes, behave when hunting their pray). This means Marjorie faced Warren quickly and spoke without worrying about his reaction or sentiment towards her comment.
