First pass thought was that maybe the blue sparks were the emperor
discharging life force from Luke.  However, when Daddy Darth
picks the emperor up, the sparks continue and Luke is no
longer in circuit. Still, that could be that the emperor was
collecting the charge from Vader and hastened his demise.

The Force pervades all and is generated by life.  It would seem
to me that Luke and Darth would have a large amount of force and
it would be just the thing for a nasty ole emperor to absorb.

As to why he didn't kill Luke outright with one discharge --
he's a sadist!  He's an evil, rotten, mean, no-good louse who
probably doesn't write home to mom.  He was taking his time and
enjoying the whole thing.
Not to mention the fact that Luke was not exactly co-operating.
--
"The soapbox of Gene Spafford"