A new-to-me spin on Kayin and Hevel

Our torah-study group has been in the fourth chapter of Genesis for a few weeks, studying the rivalry between Kayin and Hevel and the first recorded manslaughter (or murder, depending on whom you ask).

The torah text tells us that Kayin brought an offering of grain and Hevel brought an offering of the best of his flock, and God accepted the latter but not the former. (How this acceptance was communicated is left unstated.) Kayin gets pouty, God says "be careful about that", and then the text says "and Kayin said to Hevel... they were in the field (etc)". What did Kayin say? There are various takes on that, most of them leading to an argument that leads to the killing. Whether the killing is intentional or accidental is unclear (no one has yet seen a human die, so how much do they understand?). A common view seems to be that they got into an argument and Kayin acted hastily.

A midrash I saw on Shabbat gives a different spin on this:

Kayin said: God is playing favorites, and that's why he liked yours better. Hevel replied: heaven forbid! Mine was superior to yours, and that's why he liked it better. (Ah, the snotty younger sibling. :-) ) This started as a dispute but then they came to blows.

Hevel was stronger than Kayin and pinned him. Kayin said: if you kill me, who are you going to blame? There's no one else you can pin this on. Hevel let him up, at which point Kayin smacked him down and killed him. How did he kill him? He had seen Adam sacrifice an animal, so he took a knife and slit his brother's throat.

Well, that sure casts Kayin as more cunning and nasty than I'm used to seeing. The book I was reading (at shul, so can't easily check now) cited something like "tanchuma Yonatan", which I've been unable to find via Google so that's probably not exactly it. I found part of this midrash in B'reishit Rabbah 22:8 (thank you, Soncino Classics) -- that has the part about "who are you going to blame?" and the killing (with alternate opinions, including R. Yonatan saying the knife), but doesn't include the first part. (It records a different argument.) I don't yet know where that part comes from.

Interesting stuff. Maybe this week I'll be able to find the book again and remember what it is.

Correction from comments: targun yonatan, and the first part of the midrash I described.