Do you really want to throw a killing spell when the last person who threw a killing spell died?

Even if the Death Eaters would normally be too stupid to switch to their other lethal hexes (not guaranteed - Lucius, at least, has a respectable measure of intelligence), just now they've been instructed to apply a significant variety of countermeasures if Harry does anything. This greatly increases the probability of at least one of them thinking outside the extremely narrow AK box.

The last ones who remain have a choice to offer Harry their allegiance.

That's still of limited use from a social capital perspective. Harry will have a devil of a time getting the remaining Light champions to align behind him if his first public recruits are Death Eaters. (and if they're not public recruits, then the whole "spread the word" element fails and there's no reason for the public to believe that Voldemort came back and was defeated again)