The bomb was a self destruct device for Gero to activate if #17 and #18 turned against him, not something they were meant to use for themselves. #16 was fully under Gero's orders, being an actual robot and all, so the bomb was just one of his weapons. 17 and 18 were a threat to Gero, so the bomb wasn't something they were supposed to use.
The scene didn't seem to even bring it up at all anyway. It's not something that seemed worth thinking too hard about IMO, it's just something he could do. I didn't feel it had to be connected to that particular bomb.
If I struggled to the end of my determination, to the end of my way of life with my followers, if the result is ruin, then this ruin is inevitable. Grieve. Shed tears. But you cannot regret.