What are the connotations of "Eish Ochlah Eish"?
Ramban: With reference to the fire that Yisrael saw on Har Sinai, it refers to the Midas ha'Din that avenges the sin of Avodah Zarah.
Seforno: It implies that the Fire of Hashem will consume the Soul together with the body.
And what exactly, does "Keil Kana" mean?
Rashi and Targum Yonasan: It means that Hashem is zealous - that He is angry and punishes those who worship Avodah Zarah. 1
Seforno: Refer to 2:24:1:2*.
This is because, without Hashem, there is no existence that can compare to that of Hashem, and that consequently, someone who worships any other god, is ascribing to it what belongs to Hashem alone, and is akin to a woman who gives to another man what is designated for her husband exclusively.