When Thad picked up the cosmic staff he wasn't trying to steal with it or even analyze it, he just wanted to know if it would let out a single energy blast for him. He regretted even touching it as soon as it happened. Maybe it was interference from his Speed Force, and the staff's own properties, or maybe because the staff made a choice or it's conditions for the user but it backfired on him painfully.

Instead of the energy going out off of the staff it went into him, the light seemingly stabbing into his body, burning and crushing him from the inside and outside. You saw it it through his clothes as Thad tried to let go, as he started bleeding from his nose.

Then his ears before he fell to his knees, the actual user finally got of his hands before he collapsed in front of the two of you.

Laying on the ground more blood trickling from his mouth as he struggled not to throw up as his speed healing struggled to work