She was clearly looking for the artifact herself. Why? Did she want to deinfest herself? Doesn't seem so when she could have instead worked with Raynor to collect the pieces. Was she trying to keep this powerful anti zerg weapon out of other's hands? Plausible...but how about things like psi emitters? Or Protoss planet cracker class motherships? She was fine with those things running around unchecked for four years. How did she even learn of the artifact's existence?
She wasn't searching for it, she was trying to take it from people who had already found the pieces. It's different from other weapons. It can't be replicated, it can't be rebuilt, reverse engineering xel'naga technology from it could result in endless possibilities. And (perhaps aside from zeratul crashing a mothership fused with dark void energy into her like tassadar did) is the only thing she knows of that can outright stop her. She might not have even known exactly what it was made to do, but any xel'naga technology could have devastating power. It can be assumed she learned whatever she did know from investigating Xel'naga ruins like Zeratul had done and intel gathering on the terrans and protoss.
Dr. Emil Narud of the Moebius foundation, for what his expertise is worth, said that she fears the artifact's powers. The most logical motive is that she simply wanted to keep it safe so that it couldn't be used against her.
