Considering we've seen DCEU Superman show his super-duper keen senses and enough speed and reaction time to have easily blitzed the wheelchair bound Far-Right White terrorist and taken him outside and high enough where the explosion wouldn't have hurt anyone, it's clear PIS and CIS as to why everyone died in that scene.
No, it's actually the real reason. Snyder wanted a darker, tortured Superman and needed a reason for people to dislike him. Problem is Snyder isn't exactly the most gifted script writer out there, thus this stupid explosion scene.
I still think he has some dirt on Zack Synder and/or the producers, because there's absolutely no way Eisenberg did a script reading and people thought "Yes! That IS Lex Luthor!". Zero chance.