update: The current edition of Emes L'Yaakov says that the source of this idea is from the Alter of Kelm Chachma and Mussar Vol 2 page 345. It is there almost word for word. The interesting thing is that this was part of a long letter the Alter of Kelm wrote to Baron Rothschild thanking him for his efforts on behave of frumJews. He was in essence using this as a means of praise. If you see that even wicked people who intended only to kill Jews but nevertheless received reward for the resulting kiddush Hashem then surely you who have intent only for good will receive reward for the kiddush hashem that results from your generosity. I haven't found this idea anywhere else.
Rav Yaakov Kaminetsky discusses the mitzva of kiddush hashem in Parshas Emor. (Vayikra 22:32) He notes that it is not a mitzva to sancitfy G-d's name but rather that it is a mitzva that G-d's name be sanctified. From this he learns that children are also obligated in the mitzva of kiddush Hashem and this is justification for the view of some rishonim that children are to be killed rather than allow them to be converted to Christianity.
He adds [Bereishis 27:40] an additional consequence of the mitzva being that G-d's name be sanctified. He claims that the non-Jew who kills the Jew gets reward also for bringing about kiddush haShem. He uses this to explain the gemora in Sanhedrin 96b which describes a number of people such as Haman and Sancheriv whose descendants became Jewish. He says that was the reward for bringing about a kiddush HaShem - even though this was not their intent. He also claims that Yitzchok encouraged Esav to attack Yaacov so that Esav would be rewarded by the resulting kiddush Hashem.