In the following dialogue, and in general, is 奪う a synonym of 殺す? Maybe a more 'poetical' one?
Character A: 誠士郎【せいしろう】を殺したのは鈴爾【れいじ】と雪也【ゆきなり】さんだ。
Character B: なに？誠士郎【せいしろう】を奪ったのが鈴爾【れいじ】と兄さん…！？
Thank you for your help!
No, but sometimes, it can be replaced.
to steal, but in this case, it means to steal life.
実際to steal lifeって伝わるかわかってないんですけど
So the translation becomes:
Character A: A person who killed Seishirou is Reiji and Yukinari.
Character B: What? A person who stole his life is Reiji and my brother!?
By the way, why A said only 鈴爾雪也, though B said 鈴爾 and 兄さん?
No, 殺す and 奪う are not synonymous. But either can be used analogously to mean other things beyond their dictionary definitions. The boundaries of analogy are fuzzy.
"奪う" of "命を奪う" can replace "殺す," but not always so.
Then I believe the appropriate answer to the question should be "奪う is not the synonym of 殺す."
NG:ゴキブリを奪う（This just means "Taking away cockroach." It's maybe still alive.")
OK: "自動車事故が、私の息子を奪った" "The car accident took away my son forever.= He died by the car accident."
almost NG: "自動車事故が、私の息子を殺した" (If it is used as mandarin or literally, it's OK. However if you'd like to make mandarin sentence for this kind of situation, you should say "自動車事故が私の息子を奪った.")
The following example of "奪う（奪った）" doesn't mean "殺す（殺した）".
"親友のXが、私の彼を奪った" = "My best friend X stole my boy friend. (She (He) never killed him!)"