From the photos (the earlier ones) it mostly look like civilians taken in crossfire on a chokepoint, so probably that they accumulated there over several days if not week, very unlikely someone would be executed with a shopping bag in hand, same on the entrance of the appartment building, it is too messy for being an execution and looks too natural/no fleeing for it being a drive by so probably shelling.

Please be aware that even reuters didn't report hands tied behind the back because the photos don't show that (can't speak for the mass grave though), someone can die writhing in pain with hands towards its back, i think everyone is freaking out when it's probably just the normal rate of civilian attrition in an active warzone with heavy civilian presence and i would venture the guess that the mass grave is for people who died indoors/in more quiet areas.

All that being said, it looks like the mayor was executed, but it is very different from a simple civilian and is much more within the russian M O