Here is a link to a good essay by Richard Doerflinger about assisted suicide and the slippery slope. Doerflinger’s essay is a response to a recent editorial in The Economist supporting the legalization of assisted suicide. Doerflinger explains that the experience with assisted suicide in the US illustrates that opening the door to legalization typically leads down the slippery slope to broader killing.
John Keown has also emphasized the dangers of the slippery slope. Keown’s book–Euthanasia, Ethics and Public Policy: An Argument Against Legalisation (2d ed. 2018)–makes the argument with great force. Here is a link to my review of Keown’s book.