Both of you are right,
I've been wondering about something, if Eto said that Ken is so similiar to her could it be that she is to looking for the "glorious death" (witness me) damn I really want them to be together So I hope that Ken didn't really killed her (nooo) ?
Another thing is, If he(Ken) want to get rid of Ui then all he should do is to throw him off the building (before Washuu get's there)
If Ken decide to stay then Im pretty sure that Washuu will stop Ui from attacking him (he saw his potential, also now that Ken become ruthless they can get along quite nice)