Yes.
I do not pray for the survival and advancement of evil.
I pray against that.
You asked for prayers for a muslim slain by his own belief system.
To what end?
If you think that God will have mercy on evil, you are self-delusional.
What exactly do you exhort non-muslims to prayer for, regarding this typical example of muslim justice?
When advocating the denial of mercy to those trapped in an evil system, you sink to the level of evil.
Everyone deserves mercy. God sent his son to save the sinner, not the saint. In the case of this story, the boy's family are not the offenders, but the victims. Read the Gospels and the writings of the martyrs for more appropriate responses to evil.