When one who stands the Long Watch falls in battle his gene-seed is taken (if possible) to be returned to their Chapter: Flesh Tearer gene-seed goes to the Flesh Tearers, Minotaurs gene-seed goes to the Minotaurs and so on. The equipment (black and silver armor, Bolter and more) returns to the Watch Fortress in question.
Given that the Marine in question was a World Eater, I assume that he was a Black Shield. These are Space Marines who have abandoned their home Chapter to serve with the Deathwatch in perpetuity. Still there is the possibility that this was a mere ruse by an infiltrator (unlikely, given that he's a World Eater) or that he was the genuine article, a World Eater who wants to atone for the crimes of his Legion.
The problem lays with what happens when a Black Shield passes away. Many take up the emtpy shoulderpad so they can at least be honored for their last duty to the Emperor, but what happens to their gene-seed is unknown. Possibly it is interred alongside them, but when the Watch Commander finds out what the origin of this particular member is he might have the body incinerated, jettisoned or shot into a star. The armor might be kept and the Heresy-era bolter inspected, purified and made a relic of the Watch Fortress (use this as a new relic, you're welcome). Or it gets kicked out as well.
Another option, if your party wants to attempt a risky Double Henderson, is to extract the gene-seed themselves and return it to the Legion it originated from. This entails tracking Angron down and presenting him with the gene-seed of a long-dead son of his, which will likely result in him performing a TPK... unless the party is awesome enough on their way there and Angron commends them for their skill.