We can read this quote as relating primarily to Gene's relationship to Finny in A Separate Peace. This relationship is central to the story of the novel and is also at the root of the novel's themes of change/growth, innocence and the loss of innocence, and loyalty.
The love mentioned in the quotation should be read both as a general comment on important relationships and also in specific reference to the bond of friendship shared by Gene and Finny. The two are profoundly connected. They are best friends. Yet, the relationship sours when Gene suspects that Finny is competing with him and trying to keep him from doing well in his studies.
The friendship and love that is shared between the two of them is lost when Finny falls from the tree. To some extent, the love is revived, but in the end Finny dies and any possibility of a full repair for the relationship is lost.
The violence mentioned in the quotation should probably be read in reference to Finny falling from the tree andhis fall down the stairs at the end of the novel. One fall, of course, leads to the other and so we can say that each of these violent falls is responsible for Finny's death.
The quote becomes somewhat ironic when we see that the love Gene felt for Finny and the pain, guilt and regret he feels for Finny's death are actually lasting for him. Years after the events he returns to the school to purge himself of these memories, which are very real to him and lasting.
However, there is no sign of the events on the school grounds. The history between Finny and Gene that is recounted in A Separate Peace has effectively been erased by time. The tree no longer stands out to anyone but Gene.