This is the entire text of Kris picking the seed packet:
* (It's a broken container for a golden flower.)
* (You dug through the soil.)
* (In the soil were two things...)
* (One, a packet of seeds, from the golden flower.)
* (... they are covered in warning labels.)
The narration pauses a little when Kris notices the warning labels. Then, they take the seeds without hesitation. To me, it seems that Kris was looking specificaly for the shelter code, wasn't expecting to find the packet, and only decided to take it with them when they saw the warning labels.
And, what kind of warnings could they be? Well, being flowers seeds, it could be about how fragile they are, or about being an invasive species... or they could be poisonous. Yeah, with enough imagination you could come up with a thousand more, but how likely it would be for them to influence the story in comparison to warnings about their toxicity?
Also, anybody remembers how that one kid in Undertale died?
* (It's a pie wall. Seems like there's a slice of buttercup pie...) [Flavor text when investigating the pie wall at the diner]
And why would Kris pick poisonous seeds? Well, one of the possible answers is quite simple: Kris is working for the Knight, but the Soul that controls them is not. If it could pose a danger to the plan, maybe Kris has been tasked (or decided) to end their own life before we ruin everything. But, let's think again about Chara. Chara killed themself mainly to allow Asriel cross the barrier and free monsterkin by collecting more human Souls. But Chara's death was a slow, painful one. One that required once and again and again to eat a huge quantity of buttercups until they finally passed away. Why not choose a quicker method? Even if they were not afraid of death, the little crumbs we got from Chara's personality was that they were obsessed with efficiency. Why choose such an unefficient method? If they wanted to release monsters so hard they were willing to give up their life, why wait for more monsters to suffer, to fall down, to lose their families while they slowly wither away? The reason is Asgore. Chara mistook buttercups for cups of butter and accidentaly poisoned their adoptive father. He got very sick, almost died. And although Asriel says they "laughted it off", it's clear the incident affected them deeply. The Underground is not a safe place, they could have chosen any other method, even ones that could be passed as an accident, much more quicker. But they *specifically* chose poisoning, by the *same* plant they gave to Asgore. Almost as if it were a punishment.
Now, let's think again about what happened at the end of Deltarune ch5.
Asgore is kidnapped. Kidnapped, by the Knight. Kidnapped, by the person Kris is working with. Kidnapped, for staying in the Dark World Kris has just created. Kidnapped, because Kris was not strong enough, quick enough, brave enough, to save him sooner. Asgore is kidnapped, and probably, pending for sacrifice. He's going to die, and it's Kris' fault. Just as they're at fault for putting their mother at danger *twice*, for sacrificing innocent townspeople, for betraying their friends. For making that promise. It doesn't matter Kris is doing this unwillingly, it doesn't matter they are being manipulated, it doesn't matter they've done whatever they could to stop it. In their eyes, they are guilty, and now they must walk home alone and spend the entire night thinking how the dad they played cowboys with when they were a child is probably going to die soon and his blood is stained on their hands.
And then they see a packet of seeds. It's covered in warning labels.