Let's round up together what we know about Katherine's return in season 2:
- She comes back in Mystic Falls and kisses Damon, tricking him to think she was Elena;
- She stabs John Gilbert after he tells her why he hates vampires;
- When Elena comes home and finds John bleeding on the floor, she and Katherine get into a fight;
- Katherine appears to "meet" Bonnie and threatens her and then tries to kill her;
- She makes out with Damon in a wild and pasionate way;
- She tells Stefan that she loves him and when he tells her he hates her, she stabbs him;
This is all we know up to the moment! I hope this round up was usefull for all of you who want to get spoiled!