The quotes that Friar Lawrence states when introducing and implementing his plan are, by far, the most damning.
When Friar Lawrence introduces the potential plan in Act 4, Scene 1, it is clear that he is responsible for dreaming up the idea:
If, rather than to marry County Paris, Thou hast the strength of will to slay thyself, Then is it likely thou wilt undertake A thing like death to chide away this shame, That copest with death himself to ’scape from it. And if thou darest, I’ll give thee remedy. (lines 73-78)
In the above, it is clear that Juliet is only coming to seek solace and a solution. That the friar suggests such a dangerous plan is entirely his doing, as is shown in the quote above. It is he who suggests a plan that will render Juliet in a stage "like death."
Likewise, when the plan starts to go awry after Friar John cannot tell Romeo about Juliet's true condition, Friar Lawrence likewise admits guilt, stating that Juliet will be upset to hear that the plan has gone poorly:
"She will beshrew me much that Romeo / Hath had no notice of these accidents." (Act 2, Scene 5, lines 26-27).
Here, the friar realizes that he is to blame for any mishaps, and acknowledges that Juliet will be angry over the outcome. Later in the same act, he states that Juliet is shut up in a tomb alone because of his mistakes (line 30).
Thus, in the creation of, and implementation of, the plan, Friar Lawrence admits to mistakes that ultimately bring about Romeo and Juliet's demise.