>>533918795
I don't think Kris is fully on the Knight's side because of how nonsense their plan to kidnap Toriel would've been(see Andrew Cunningham recent vid).
It seems more reasonable to assume that the Knight was aiming to kidnap Toriel, so Kris lured Undyne to the house to get kidnapped in her stead.
It also explains why Tenna brings up his deal with Kris separately from his "deal" with the Knight in the sword route. He was told by Kris to keep the FUN gang distracted until Undyne showed up and to pretend this was the Knight's plan.
Note how Kris deals more damage to Tenna if you do the sword route. The natural assumption was that Kris gave into their violent urges like ERAM suggested. But I think they actually were super pissed at Tenna for leaving his 8-bit self mini-diary in the 3rd secret board revealing something that wasn't supposed to be seen by the player.
Like, Kris is plotting something behind the Knight's back that they absolutely can't let the player know either or it would ruin their plan.