You see, that pixel was a very important pixel. Being a part of your head, it probably stored vital circuitry, and it melting could (and, apparently did) cause a system shutdown.
While, in game terms, it can feel pretty ridiculous that you were doing so well, and you die from what seems like pretty minor damage, what else do you expect the developer to do? Just how many vital components should your robot be able to lose before they shut down?