A new study by Salazar et al. in The Journal of Clinical Investigation demonstrates that THC causes tumor cells to begin to degrade themselves from the inside (a process called autophagy, i.e. "self-eating"). Although autophagy has been shown to promote cell survival in some cases and cell death in others, the authors show that in this case it causes cancer cells to undergo programmed cell death (apoptosis). Thus, THC activates a series of events within cancer cells, inhibiting tumor growth.
Despite the imagery that the title of this post (and maybe even casual experience) may imply, it's not that THC causes cancer cells to ravenously and indiscriminately destroy themselves. Rather, by binding to certain cell-surface receptors, THC activates a cell signaling pathway that leads to cell death. This is a very orderly process, with autophagy just contributing one important step. The authors of this paper work out many of the molecular players in this process, using siRNA and cells derived from knockout mice, although I'll leave it to the interested reader to further explore that aspect by reading the paper if so inclined.