Tyrande's decision is a farce. She chose constantly to pursue her completely rational and justifiable grievances against Sylvanas. She speaks it out loud in the fight cinematic, accepting that she might lose her life but she'll bring Sylvanas down with her. At that point Elune pulls out the power cord and she loses. After that point Sylvanas is out of reach and she's on the verge of exploding. It's only at this point and with Tyrande having an out of body experience and verging death that she presents her with either choosing renewal, i.e a realm that's basically fine already, or 'vengeance', i.e the defeat of someone who nearly eliminated her race to feed Satan. Tyrande later speaks of all of it as an out of body experience that she doesn't remember Elune's plan for despite Elune saying there was no plan and she just fucked up the delivery.