Question
Was Daśaratha/Dasharatha at fault to receive a curse for killing a boy unintentionally?
Answer
It is mentioned in Valmiki Ramayana that Dasratha was an ace hunter who could shoot unseen (just on hearing ) Ayodhya Kanda Ch 63 Verse 11
"This sin was done by me while I was young and wielding a bow. At that time, I was famous as a young man, who can shoot heard (but unseen) object by an arrow, O Kausalya!"
But he mistakenly killed the young ascetic taking him for an elephant drinking water.
As the act was unintentional , he didn't incur sin of killing the ascetic. But as Dasratha caused suffering to ascetic's parents, ascetic's father cursed him
This is evident at the time of deceased ascetic's father lament
Valmiki Ramayana Ayodhya Kanda Chp 64 , verses 23-25
"A killing brought about by a warrior premeditatedly and in particular to a hermit, would expel even Indra from his post."
"The head of a dispatcher of a weapon with a knowledge of the matter on a sage established in austerity or such an unmarried student practicing sacred study, gets burst into seven pieces."
"You are still surviving because you have done it unknowingly. Not even you, but today the whole of Ikshvaaku dynasty would have been no more, if it is not the case."
As seen from above verses the ascetic father acknowledges that it is an unintentional act . If it wasn't huge calamity would have befallen on Dasratha.
Ascetic's father acknowledges that Dasratha doesn't incur the sin of killing because he acted out of ignorance
O, king! Since the sage was killed by you, belonging to the warrior class, through ignorance, the sin of killing a Brahmin will not take possession of you
Manusmriti 11.46 says
A sin committed unintentionally is expiated by the reciting of the Veda; while that committed intentionally, in folly, is expiated by the various forms of expiatory rites.—(46)
Because Dasratha was safe as far as sin is concerned (all he had to do was recite vedas), the father of ascetic took matters in his own hands .After all he was in pain and agony because of Dasratha
Verse 55
O, king! Even as I am suffering from agony now because of the loss of my son, in the same manner; you will also die due to agony caused by the loss of your son
Finally, there is no past life Karma in play here. If there was Prarabdha karma then he would suffer the loss of Rama without the curse. Dasratha suffered in the end solely because of the curse
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