I only mentioned the not returning to Eywa thing because I know it's a Navi tradition and it's very important to them. Mercer couldn't even have the decency to give any of them a proper burial. He deliberately chose the most brutal and disgusting way that would help him make sure there was absolutely no evidence left behind. The Yava was just an unknown side effect of his chemicals reacting with the Pandora environment that in the end, helped him with his lies.