It is not the thief who is hanged, but one who was caught stealing.

Related:

• He who holds the ladder is as bad as the thief.
  German...
• He who steals once is always a thief.
  Spanish...
• Criminal:
  One who gets caught...
• Who punishes one threatens a hundred.
  French...
• He who steals for others ends up being hanged for
  himself...
• There is one who kisses, and the other who offers a cheek.
  French...
• The big thieves hang the little ones.
  Czech...
• Set a thief to catch a thief.
  French...
• Men are not hanged for stealing horses, but that horses may not be stolen.
  Lord Halifax...