I don't think mc with below IQ 100 can suddenly become mc with IQ 200 after witnessing his parents's death. First of all, if he tell his parents about system and help them or use wisely(don't use lazily), they would not have to die. In this novel, there is no need to revenge around because the cause of his parents's death is literally his naivety. Mc is ungrateful upon his nice parents.
Adui
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LIKEWhat use of his feeling after their death. He should have cherished when they are alive. You said he has no feeling until they die because he was transmigrator. Then didn't he see they worked days and nights to support him when he was lazily hunting and wasting his times. He had a too op system and he had to do is conquering some iron and bronze dungeons to have limitless supplies to support his family. But he never give them anything for what they providing for him. Even if he has no feeling, he transmigrated into their son and had memories so why didn't he repay them? It is not ungrateful then what is?
Single_Boi:yo he was a transmigrator with no feeling for his parents until after their death.