Armsmaster did mention when confronted about what he did that his combat analysis software had issues with to many variables, which was why some hero’s died, so the reason he wanted to fight Leviathan alone may have been to increase the effectiveness of the analysis software. In that case it isn’t quite as bad as he would believe that by sacrificing some villains he could kill Leviathan and that if he didn’t the combat software wouldn’t have been good enough.
Ehhh. Maybe. Not sure I buy it, but I suppose it works.
…I guess it would’ve made sense for him to want an audience if they wouldn’t get in the way. Hm.
But yeah, either way, it takes us back to arrogance and faith in his own work. He’d have to legitimately think his system was so good at peak performance that it working just a little better was worth getting rid of all the help from others, at the cost of lives.