I don't think Ronnie knew what she was doing when she swapped her baby with Kat's, she was in a confused state. When she first took him she was seen telling Kats baby that she would take him home soon she just wanted to spend more time with him, but when she finaly went to take him back Jack turned up in a taxi, and held the baby who he believed to be his son. He said "It's just the three of us now" and that is all Ronnie ever wanted. While this was going on she heard Kats screams and knew the Slaters had already discovered her baby in the cot. Now Ronnie feels trapped and doesnt know what to do for the best, she probably feels that things have gone to far and there is no going back from what she has done.