Main reason he died was because they didn't have the time or resources to diagnose and repair him. The cause for his blondless could've been a mechanical failure in the eyestock, vision loss for the mutant inside, or brain damage preventing the eyes vision from being processed. It was a liability to the mission, and since these are Daleks, ya know the rest.
So the Dalek ray gun can kill something literally instantly, but Daleks always dial back the power just slightly so as to ensure that the target will suffer first. Because they’re bastards like that. I wonder if that applies to when they’re exterminating a Dalek that has, though no fault of its own, become a liability and so requests extermination. Do they still make it suffer, or do they crank it up to 100%?