The reason for this was that if one slice ('hito-kire' which also means to kill someone) of takuan was served the samurai would become angry and said, 'Are you trying to tell me to kill someone,' and, if three slices ('mi-kire' which also means to cut oneself or to commit harakiri) of takuan was served the samurai would also become angry and said, 'Are you trying to tell me to commit harakiri?'
それは、沢庵を一切れだけ出した場合、侍は「一切れ(人を斬れ)と申すのか!」と怒り出し、また、三切れ出した場合は「三切れ(身斬れ、つまり腹を切る)と申すのか!」と怒り出すためである。 - Wikipedia日英京都関連文書対訳コーパス