Coming into the proximity of the Qodshei Qodshim requires protection . For Aharon, that meant a proper Q'toret. An improper Qktoret - even b'shogeig - could have left the Kohen Gadol without his "radiation suit." He would risk death through exposure to an overwhelming dose of Q'dusha, not through his transgressions. Like an electrician with a faulty rubber glove, the shock would be overwhelming.
Back to Nadav and Avihu. Fire consumed them mainly because their ersatz Q'toret failed to protect them. Therefore Aharon was commanded how to avoid such a similar catastrophe.
And as for Uzah - in the Haftara of Sh'mini - he wasn't punished. He lacked protection from the aron's overwhelming Q'dusha.
Electricity, radiation and high places all entail physical risk. It's like a child sticking his finger in an exposed socket. Hashem is not punishing the kid. We are fixated on the concept of seeing din as punishment. Din isn't "punishment," but natural consequences.