This is a part of Undertale that I think gets glossed over a bit too much in some comics involving Frisk. As a video game protagonist, Frisk dies repeatedly, especially if the player is unskilled.
Some people portray their journey through the Underground as a fun, lighthearted thing that they'd willingly repeat just because they enjoyed it so much, and while that may be true for the player, I doubt the young kid who gets stabbed, burned, electrocuted and/or beaten to death multiple times would feel the same way.
Not that Undyne knows what she put them through, given that she loses her memories every time Frisk dies and reloads a save; for all she knows, they survived the battle on the first try, with relatively little effort. If she knew how much she'd traumatized them, she'd be far less cheerful about it.
As for whether or not that trauma was a contributing factor in their decision to go from cute, naive kid to stony-faced serial killer in timeline 4, you'll just have to wait and see.
I thought about keeping Frisk's eyes closed through most or all of the comic, but while I was drawing the second to last panel, I concluded that the effect the open eyes would create was a temptation not to be resisted.
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