Obviously Kakashi has way more fighting experience and a variety of jutsus, which would likely overwhelm Killua and make him hesitant of making a move. However, I feel like they both grew up in similar ways with assassination being of utmost priority, following rules, and experiencing difficult lives at an early age. I think if Killua were to flip on his assassin mindset after having Godspeed/ overall hatsu developed, he could surprise Kakashi and win

1