Let death take them away. Let them go down alive into the realm of the dead.
One may view this prayer as a continuation of Psalm 55:9. Why does the author pronounce this curse? He does so because evil dwells in their home—yes, in their heart. It shows that evil controls their whole life. Evil is completely at home there. And when the evildoers die, their misdeeds finally stop.
This is not about revenge from the author, but about justice. That is why this is spoken as a prayer or a wish. Think of the wickedness of Korah and his family in Numbers 16:33 So they and all that belonged to them went down alive into Sheol, and the earth closed over them, and they perished from the midst of the assembly.
15 Let death steal over them; let them go down to Sheol alive; for evil is in their dwelling place and in their heart.