# to melt their hearts

"to fill them with terror" or "to cause them to be terrified"

# to multiply their fallen

"to kill many people among them"

# set the sword for slaughter

"placed soldiers armed with swords at their gates, ready to slaughter the people"

# their gates

The word "their" refers to the people of Jerusalem.

# It is made like lightning

"My sword is polished and flashes like lightning"

# it is grasped for slaughter

"released to slaughter the people"