Is God a moral monster for giving efficacious grace to some and not others?
Efficacious grace normally refers to the grace that God gives to infallibly secure that one will choose him and be saved. This grace is entirely the result of God’s free will independent of man’s choice.
But it doesn’t follow from this that those who end up in hell are there due to some injustice on God’s part. God gives every individual the grace needed to obtain salvation (2 Cor. 12:9). Therefore, all who end up in hell are there as a result of their rejection of God’s grace.
As to why God infallibly attracts some souls and not others by his grace and thus secures their salvation is ultimately a mystery.