First time I encountered a spot like this. So it's quite rare.

V1, $2100 eff limps in MP1. V1 is a recplayer. Not tight. Spews moderately postflop. Tried to semi-bluff raise me earlier OTF (3-way flop) with a draw. So there's definitely some aggro in him.
V2, older recplayer makes it $30
V3, tight guy, seems solid, possibly a grinder 3bets to $90 in HJ
V4 OTB, solid, generally tight young grinder 4bs to $150 OTB Has about $1600, I have him covered
SB folds. I 5-bet to $450 in BB with KK.
Folds to MP1, who 6-bet shoves $2100 eff.

Folds around to you. You and why?

Also, I'd like to use this as an opportunity to discuss a more general topic of: "when is folding KK preflop correct?"