It seems that God is saying, "Believe in me and you will be rewarded, but if you do not you will be punished eternally." This makes God seem like a slave master and contradicts his all-loving nature.
It’s true that if we die in friendship with God, we’ll be rewarded with the gift that he has promised, namely, heaven. It’s also true that if we reject God and die outside of his friendship, then we’ll be punished forever in hell.
But it doesn’t follow from these things that God is a slave master or that we are his slaves. The eternal reward of definitive happiness and the eternal torment of hell are intrinsically tied to how God made us.
Given the nature that he created us with, a nature capable of knowledge and love, we can achieve perfect and complete happiness only in him, who is infinite knowledge and love itself. This perfect happiness is granted to us in heaven when we are confirmed in our choice to be united with him after death.
But if we choose not to be united with him, and we die making that choice, then the only possible state of existence is definitive separation from God. And given our natures as rational beings made for God, such separation from God can only be a definitive state of unhappiness.
As the Catechism of the Catholic Church states with regard to the punishment of hell, “[It] must not be conceived of as a kind of vengeance inflicted by God from withou, but as following from the very nature of sin” (1472).
For more information on this topic, see my article “Doesn’t Hell Make God an Unjust Tyrant?”