Whenever the potentials have been in danger before, Kennedy has always been the first (and only) to jump into the fray to protect her sisters. She seeks responsibility, because she is self-sufficient. She believes in her own abilities.Her scenes in Chosen (being Willow's "anchor" which should've been Xander's role given her friendship, and taking the scythe to Buffy) are basically just her avoiding the majority of the fight. She practically came in at the end of it right before Spike finished the job. She wasn't there from the start. and so avoided the vast majority of the bloodshed
And Kennedy clearly cares about Willow. She isn't just a means to an end. She helps Willow when she is clearly in over her head in "Killer" and does her best to support Willow's confidence throughout the season.
I think the reason Willow gave for breaking up with Kennedy was insincere. Kennedy never seems as though she only cares for magical super-Willow, and neither did Tara, but Willow projected her insecurities onto her too. Kennedy is pretty darn loyal, both on the show and beyond.