I will be waiting…
It was one momentary mistake.
His wife was crying, he had gained new powers, the rest of his wives were worried as well. He had to not only test his new powers but first and foremost, he needed to reassure his crying wife, then he needed to reassure the rest of his wives, there were too many things in his mind, his thoughts were distracted.
And because of that, making a small mistake was completely normal.
Anyone could have made such a mistake.
However,
A mistake was still a mistake.
No matter how 'common' it was, no matter what circumstances or situations led to it, you cannot possibly leave a dangerous beast with unknown powers without a leash, a mistake like that was lethal.
Especially in this case, where this mistake could affect what Nux cared about the most.
His wife.
And not just any wife,
His first wife.