The hell was wrong with Phoebe?
She knew full well that Tommy and Malina were the only ones who were able to prevent the incoming disaster, and save everyone (including her) so she gets the bright idea to kill Malina, and doom everyone (including herself and Quentin)! Ok, fine, I get it! Phoebe is a stupid evil idiot, but there is still more to it than that!
She held Malina in the air, so if she wanted her to die, she could have dropped her. Malina falls to her death, problem solved, right? Wrong! She did drop her once, but immediately caught her and lifted her back up. Why did she did that? Note that Malina failed to grab the railing the first time, so she would have died, if Phoebe wouldn't have caught her.