If a person plans to do something that they know is an aveira, should they daven to be successful?
I think one of the main things this question is exploring is how a person should relate to God vis-a-vis purposefully wrong actions.
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The Rashba”tz, in Magen Avot on Pirkei Avot, chapter 21 says that even a sinner have in mind “leshem shomayim” and brings the example of the thief who prays that Hashem should give him success in his breaking-in to the house and the oath that Saul took in the name of Hashem when he enquired of the familiar spirit in Ein Dor and another example.
הרשב"ץ קובע שגם אדם חוטא יש לו לכוון לשם שמים, וכדוגמה הוא מביא את סיפור הגנב המתפלל לה' שיצליח בפריצה שלו, ואת השבועה שנשבע שאול בשם ה' כאשר ביקר אצל בעלת האוב בעין דור, שמדומה במדרש תנחומא לאישה נואפת שנשבעת בחיי בעלה
It seems therefore that one should daven for success in an aveira, amazing as it sounds.
1. Found in a footnote to the Wikipedia article on Aveirah Lishmo
There is a Baal Shem Tov story about a robber who got brachos from the Besht. When questioned about why he associated with the thief, the Besht replied that the commandment "make for youurself a rabbi" (pirkei avot) was not only given to the rigeous. Even the wicked need rabbis.