Evidence it was by accident:

“I reached out but you were gone. You went down through those little branches underneath and when I reached out there was only air.” chapter 5

“what happened there at the tree, that goddamn tree, I’m going to cut down that tree, who cares who can jump out of it”

“it was the first clumsy physical action I had ever seen him make”

“He had never been jealous of me for a second. Now I knew that there never was and never could have been any rivalry between us”

ON PURPOSE

“I deliberately jounced the limb so you would fall off”

“If anyone had been suspicious of me I might have developed some strength to defend myself but there was nothing, no one suspected”

“I did not cry then or ever about Finny”

“he must be too sick or too noble to tell them”

“trying to empty my mind…to forget who I was”

“I cried because of kindness which I had not expected”

“Finny his balance gone swung his head around to look at me for an instant with extreme interest, and then he tumbled sideways”