The best source of krill oil is eating fresh krill.
If you are taking an omega 3 supplement, it is probably at least partially rancid before you take it. If you do take it, it almost certainly oxidizes inside you. You couldn't pay me to take omega 3 supplements. If you think soybean oil is prone to rancidity, you haven't looked into EPA and DHA. They make soybean oil look like coconut oil in terms of resistance to oxidation.
The Great Fish Oil Experiment
That's why I don't supplement omega 3's. I eat fish. Why not just eat some fish every week? It's delicious and not particularly expensive.