So... no one has a potential plan? At this point the only alternative I can think of is assume he's the honourable type and try to challenge him to single combat in the armoury... or mad lib and hope the Shadowseer appe--... is this the Shadowseer?
Hmmmm. Tell him the Nails are biting into his thick skull, and if he disapproves of your choice to grab explosives, he can show it off in a duel. Challenge him to a fight, to first, second or third blood, and if you have to, throw the fight, to let him feel like he's won. If he does enough damage, you can let them move on ahead, and sneak off after.
I am still in the warp and waiting for a post from @Maensith , although I will probably arrive with the necrons. @all : I call dibs on the looted terminator armours. Most of you cannot use them anyway.
Implying we aren't going to melt down every last traitor corpse we can, and toss the remains out the air lock.
Implying that there would be something left of them to melt down after they had a meeting with Necron weaponry
Damn, if you Necrons ever get over that whole "Get off my damn lawn, you whippersnappers!" thing, I would mind having you as a unit of guass janitors. Be a nice way to eradicate every particle of a mess.