How people react to Mortimer will probably depend on which side of his "operations" they were on. The soldier types, for instance, probably wouldn't like him very much since he tends to oppose militaries more often than not. Especially the larger ones. They might see him as a terrorist or a monster or some sort of apocalyptic level threat (though he's only ever destroyed one planet, and it was an uninhabited automated manufacturing world at that. And he really only played a small part in tipping it over the edge...).

Non-military types, however, would probably remember him more fondly. Or at least the legends of him. After all, a guy who's been around for over a thousand years is bound to have a lot of those, so feel free to make some up if you feel so inclined.