>>121783328You're not asking the right questions, OP.
The right question is: Why didn't he let Steppenwolf into the death room where the Motherbox was kept, and then press the button? Remember, dude was completely vaporized in there. There was probably a chance that it would have, if not kill Steppenwolf, at least seriously injure him.
Imagine if the main villain of Justice League was killed by Cyborg's dad with a simple trap that he didn't even really expect to work.