Appreciating others for what they do for us or why they do it?

In our discussion of the Chofetz Chaim, the issue of the importance of motivation versus actions was mentioned in regards to the dispute of the Taz and the Sma. The case is one in which an assailant is hitting his victim.  The only way to stop him is to hit the assailant. Is it permitted to hit the assailant in order to save the victim from a beating? The Sema says that if you typically try to help people than it is permitted to hit the assailant to save the victim. But if typically you don't concern yourself with saving victims then it is not permitted because this time you must be motivated by hatred of the assailant and not because of a desire to help the victim. The Taz says it makes no sense that you can't help someone because your motivation is problematic. He says it is always permitted to hit the assailant when that is the only way to save the victim - and it doesn't matter what your motivation is.

Rav Yitzchok Zilberstein relates the following story. There was a man - Reuven who had developed a strong hatred for Shimon. In addition he also bore a grudge against a certain yeshiva bachur. Out of his hatred he developed a plan to exact revenge on both of them. One day he told the bachur, that there was a very distinguished talmid chachom who had a fine daughter and they were interested in him. The bachur checked out the family and it was truly an impressive family and the girl was highly praised - it was much more than he had ever hoped to aspire to. Of course he said yes. Reuven told him that he had arranged everything and gave him a time to go to Shimon's house.

When he knocked and the door, Shimon opened it and was truly puzzled as to why this young man had come to see his daughter. He knew nothing about it. It soon became obvious that Reuven was behind this and that his sole motivation was to embarrass them both. Shimon being a refined individual tried to spare the young man as much shame as possible and invited him in to talk Torah over coffee and cake. As they talked Shimon became greatly impressed with the young man - not only his Torah knowledge but his middos. He decided that was in fact what he had hoped for his daughter and suggested that the young man in fact go out with his daughter.

The relationship progressed extremely well and a short time later they were engaged. 

When Reuven discovered that his evil design had been thwarted, he decided to hide his disappointment and make the best of it. He went to Shimon, wished him mazel tov and then asked him for the shadchan fees.

Shimon was outraged, but being a true talmid chachom - he told Reuven that they should go speak to Rav Zilberstein to decide what was appropriate - since obviously Reuven had no intention of making a shidduch but only causing shame and embarrasment. But on the other hand he had brought about the shidduch.

Rav Zilberstein concluded that Reuven was obviously not the shadchan and thus did not deserve the fee. "G-d made the shidduch not you." He noted that this was comparable to Bilam going to curse the Jews and ending up blessing them. Bilam is not given credit for the blessing but he had not intent to cause benefit only harm.

According to the Taz, why shouldn't Reuven be paid for what he accomplished?

update: My concern is not choshem mishpat but the mida of hakaros hatov. Do they need to show gratitude to Reuven for what happened. It seems clear from Rav Zilberstein that there is no reason to show gratitude. 
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This is a post I made 10 years ago Avodah - Gratitude towards your enemy?

R' Chaim Shmuelevitz [Sichos Mussar II #42 page 117] in his discussion of gratitude mentions Shemos Rabbah (1:32) Moshe is introduced to his future father in law as "an Egyptian"

 [Soncino translation] "alternative explanation of AN EGYPTIAN: Moses can be compared to one bitten by a lizard, who ran to place his feet in the water. When he put them in the river, he observed that a small  child was drowning; so he stretched out his hand and saved him. Thereupon the child said: =91Had it not been for you, I would already have perished.=92 To which the man replied: =91Not I have saved you, but the lizard who bit me and from which I escaped, he saved you.=92 Thus the daughters of Jethro greeted Moses: ' Thanks for saving us from the hand of the shepherds.=92 Moses replied: =91The Egyptian whom I slew, he delivered you.=92 They therefore said to their father AN EGYPTIAN. meaning that the Egyptian whom this man slew caused him to come to us.  "

R' Chaim says that we learn from this that one has an obligation of hakaras hatov based upon the consequence of the action not the motivation. Therefore even though the snake and the Egyptian had not intended good but rather the opposite - the recipient of benefit is obligated to have gratitude towards that which caused the benefit.

The commentaries on this medrash have a simpler explanation. Moshe and the rescuer of the child were merely noting that they should not be  viewed as the source of the good but rather HaShem through His various agents.

Does anybody else have R' Shmuelevitz's understanding of hakaras hatov? While the Chovas HaLevavos does mention that hakaras hatov is not dependent upon the motivation of the source - but here we are talking about one's enemy. It would follow that we need to have hakaras hatov to Amalek etc for causing us to do tshuva.