The better betta pellets (like Omega One) include salmon and halibut, which are not from a betta's natural habitat either; krill is on the ingredient list of Omega One Betta Flakes (specifically as part of a "seafood mix") so that sounds like marine krill to me. If it is fresh krill from your marine tank, I'd be worried about the salt content more than anything else. In the wild, IIRC, bettas eat almost entirely a diet of insects, insect larvae, and occasionally much smaller fish, so probably very little salt in their diet. In general, what we feed bettas (as well as nearly all other pets) is fairly different from what they eat in the wild; this is not always a bad thing because their lives in the wild are very different from their lives in captivity (i.e., they have food consistently available, they don't have to hunt, they don't have to defend themselves, etc.)
I would say, therefore, you almost certainly CAN feed bettas krill, but you may need to prepare it in a special way to be safe and suitable for your bettas. It might be more trouble to prepare it yourself than to just buy freeze-dried or frozen krill. FWIW, I offered both my snail and my betta a piece of soaked freeze-dried krill the other day; the snail ignored it and the betta nibbled it once then ignored it.