@Madnacc
In a world of swords an magic. There has been no indication so far (except the MCs words an NO ONE ELSE) that giving a 9y old a sword (regardless of what that sword is) is a bad or uncommon thing.
Except that the sword appears to be considerably more dangerous than a nuclear warhead with a history of evil deeds, with power extraordinary even within the setting (thus making it hard for anyone to control should it wish to cause further destruction)? Are you actually arguing that such things belongs in the hands of a 9-year old? And comparing it to giving a 9-year old ANY old sword?
1) you don't really "let" someone do something you are not aware of. The moment MC found out he banned that behavior.
You "let" someone do that when other adults hear about it and do nothing but laugh it off while ignoring the complaints of the victim. It's a bit like if a child throws a rock through a window and the only adult person voicing complaints is brushed off by other adults who do nothing to disencourage such behavior: they LET the child do such things.
The sword acknowledges a 'master' and has obligations to them. With that it's reasonable to think that its previous feats might not have been its own will but that of its master back then
Nope, totally wrong. What you think is "reasonable" is irrelevant: the sword itself states that in the past it has drained its then-user as well as his enemy, "because it got over-enthusiastic". Likewise, nobody ordered it to drain the life out of its surroundings, it did that on its own volition. That it feels it has obligations towards its master is not shown to be anything other than a fancy by the amnesiac and whimsical sword (as per the title: The WHIMSICAL Cursed Sword).
This isn't our world. For all we know they do that on a daily basis just to show of their strength to people. There is also no indication (so far) that this is in any way 'evil' or even 'problematic'
No, we know with absolute certainty that it's not an everyday occurrence, considering that it was a Dark God's signature move (admittedly in order to show off his power in this case). Also, regardless of ethical norms in this absurdist comedic fantasy world, it's objectively a bad idea to let pre-pubescent children run around with the sapient equivalents of nuclear weaponry.
Since he found out about his sisters 'prank' he hasn't been threatened to lose his soul even once.
Assuming she gave a single fuck about his command that she stop doing it, considering that she has shown zero regard for anything he says so far. Also, if she did indeed stop, it still does not make her possession of the sword a good (or anything less than batshit insanely mega-terrible) idea.
Personally I think he's a jackass because he doesn't listen to others and acts essentially like a stereotypical racist: "He/She/It looks shady so it must be bad and dangerous. Get rid of it."
Are you trying to troll me right now? Are you honestly saying the sole reasoning behind his judgment is that it LOOKS shady, after it's drained the life out of a huge area simply by being in contact with it, after it's pulled his soul from his body, after it's blown up a mountain, after clearly being identified as having history of terrible deeds?
I mean, how do you even argue these points despite the fact that the comedy of this series is entirely built on the premise that "In an insane society, the sane man must appear insane"?