Was Killing Vali from behind an act of Dharma?

Srimad Ramayana

Vali dismissing Tara's words starts out to fight out Sugreeva. A ghastly fight ensues in which Vali depletes Sugreeva's strength and thus Sugreeva exasperatedly started searching for Sri Rama to counteract Vali. 

 Sri Rama shoots an arrow, which hits Vali on chest, from behind a tree but does not render him dead immediately. Thus, that mighty Vali is brought down to ground.  Sri Rama and Lakshmana approach the dying Vali with due honour.

Vali questions logically about the propriety of Sri Rama in killing him from behind, even at his dying stage. Sri Rama explains to dying Vali as to why he gave such a punishment.

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Sri Rama categorically explains all the questions put by Vali from the viewpoint of scriptures that lay down principle for sanaatana dharma, eternal tradition, as well as raaja dharma, king's duty.

•  As for you, you brought virtue to a state of decadence, rendered yourself reprovable by your own decadent behaviour, for carnality alone has become your primary doctrine, and thus you have not abided by the conduct appropriate to a king.
• An younger brother, a son, and a disciple with good characteristics, these three are to be deemed as one's own sons, for such matters take base on rectitude alone.
• The Dharma sukshma practised by principled people is very subtle and highly imponderable
• Realise this reason by which I have eliminated you... you misbehaved with your brother's wife, forsaking the perpetual tradition.
• While the great-souled Sugreeva is still alive, you with your habit of sinful acts have lustily misbehaved with Sugreeva's wife Ruma, who should be counted as your daughter-in-law.
• As a Kshatriya emerged from a best dynasty I do not tolerate your wrongdoing, and the punishment to the one who lustfully indulges with his daughter, or with his sister, or with the wife of his younger brother is his elimination, as recalled from scriptures.
• People will be capturing several animals, either covertly or overtly, with snares, springes and even with numerous contrivances.
• Meat eating people will undeniably kill animals, either they are speedily sprinting or standing steadily, fully dismayed or undismayed, vigilant or unvigilant, and even if they are facing away, in that there is no sacrilege.
•  In this world even the kingly sages well-versed in virtue will go on hunting, and hunting is no face to face game, as such, I felled you in combat with my arrow because you are a tree-branch animal, whether you are not combating with me or combating against me.

Sri Rama’s treating Vali as animal was not a result of anger or out of fear of Vali’s strength but it was the result of pondering over scriptures.  The scriptures say

आहार-निद्रा-भय-मैथुनं च समानमेतत्पशुभिर्नराणाम् ।
धर्मो हि तेषामधिको विशेषो धर्मेण हीनाः पशुभिः समानाः ॥ 

"food, sleep, fear and mating, these acts of humans are similar to animals'.  Of them (humans), dharma (right conduct) is the only special thing, without dharma humans are also animals."

As Vali deviated from Dharma, by capturing Ruma, who is equal to his daughter-in-law, and should be treated as animal and hitting an animal by standing invisible is not wrong.

Vali agreed to Sri Rama's reply.  He said with adjoined palms

प्रत्युवाच ततो रामम् प्रांजलिर् वानरेश्वरः |
यत् त्वम् आत्थ नरश्रेष्ठ तत् थथा एव न अत्र संशयः ||  (Kishkinda Kanda Sarga 18 sloka 45).

  "oh, best one among men, what all you have said is that way proper, undoubtedly”

However, even today many atheists, followers of other Religions are questioning as to how can the word "Dharmatma" (one, whose mind is fixed in righteousness, and who practices righteousness in thinking, speech and in action) be applied to Sri Rama, who hits Vali from behind.

The answer lies in Srimad Ramayana itself.  Let us examine the following episode of Yuddha Kanda.

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In a terrific fight that ensued Lakshmana and Indrajit, who were engaged in releasing several streams of arrows, fought like two lions. Lakshmana the foremost among humans and Indrajit the foremost among demons fought with rejoice.

That fierce battle went on for a longer period due to exchange of many ASTRAs presided by various deities like Varuna, Agni, Maheswara, etc.

Finally fitting a missile presided over by Indra to his bow and drawing it, Lakshmana discharged it at Indrajit after making the following prayer:

धर्मात्मा सत्यसन्धश्च रामो दाशरथिर्यदि ||
पौरुषे चाप्रतिद्वन्द्वस्तदेनं जहि रावणिम् |  (Yuddha Kanda - Sarga 90 -  Sloka 71)

“O my dear arrow! If Rama the son of Dasaratha has set his mind on virtue (Dharma), keeps up his promise and is second to none in his prowess, destroy this Indrajit.”

That large head of Indrajit, separated from his trunk and moistened with blood, was seen on the ground, shining like gold.

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One can deceive people around him by posing to be a Dharmatma but nobody can fool the NATURE/GOD. 

Lakshmana used many astras during the battle to kill Indrajit, but in vain. Finally, when employing the astra presided by Lord Indra, he prayed to the NATURE/GOD, by invoking the ascetic power of Sri Rama in (i) upholding DHARMA at all times, (ii) honouring his word, for eliminating Indrajit.  

In my view, that prayer did the job.  If Sri Rama was not the upholder of Dharma, that prayer would have failed. Thus, we can infer that killing of Vali from behind a tree was also as per Dharma.

Dharma is very subtle.  It cannot be understood easily.  If we cannot understand the subtelity of Dharma, it does not mean that an upholder of Dharma did something wrong.