The small bumblebees was flying around the garden seemingly on the back of the larger black bumblebee. When it landed it looked as if the small bumblebee was frantically trying to sting the larger black bumblebee. The black bumblebee was quite still so I assume it may have been dying or already
These are not bees but flies, possibly a type of hoverfly - many of which mimic bees and wasps as a form of defence. (If they were bees they'd have longer antennae and 2 pairs of wings each - although the latter aspect may be hard to see if they are folded together).
There are loads of species so I won't attempt to id them. No stinging going on; they appear to be mating - well it is spring!