They threw gas on the vampire and then lit him on fire, which was the final nail in his coffin.
They finished the vampire off by throwing gas on him and lighting him on fire.
この場合、次のような言い方ができますよ。
ーThey threw gas on the vampire and then lit him on fire, which was the final nail in his coffin.
「彼らは吸血鬼に油をかけ火をつけ、とどめをさした」
to throw gas on ... で「…に油をかける」
to light on fire で「火をつける」
to put a final nail in the coffin の代わりに、to finish ... off を使うとより自然な言い方になります。
ーThey finished the vampire off by throwing gas on him and lighting him on fire.
「彼らは吸血鬼に油をかけ火をつけ、とどめをさした」
ご参考まで！
"To put the final nail in the coffin, they covered the vampire in oil and set it on fire."
"To put the final nail in the coffin, they covered the vampire in oil and set it on fire."
"to put the final nail in the coffin" 「とどめをさした」"To finish it off" も言えます。
"they" 「彼らは」
"covered ~ in oil" 「〜に油をかけ」
"the vampire" 「吸血鬼」
"set it on fire" 「火をつけて」"set it ablaze" もつかえます。
[火をつけて](https://eikaiwa.dmm.com/uknow/questions/103205/)についてもっと詳しく知りたい方はクリックしてください。
カナダ 
オーストラリア 
