Only 3 Guardsmen Guarding the Emperor?
Not to mention that they would kill Luke, they are far better shots than Stormtroopers.

poor luke lols.

Isn´t it bad time as well? Star Wars take place log time ago (in a galaxy far far away) while WH4OK is in the grim darkness of the far future... ;-)

And now failddon appears and kills the emperor...... cause stormtroopers are even worse than gretchin (or tau in HtH)
A glorious day for Chaos, and a bad day for the Empire (not the Emperium, the space marines will still save the day)

and after killing the emperor, humanity was left stranded in the warp and forces of chaos were free to pour out from the eye of terror and claim whatever worlds they desired.

The horrors of space would engulf the planet simply because Luke wanted to kill a crippled dude for his ego.

Seeing that he was responsible for the destruction of human race, Luke Skywalker suffers a mental breakdown, realizing he has done such evil that he never could make amends for what he did.
All he can do is wait for inquisition to arrive and accept the most horrible punishent they can devise for him or commit suicide.