But is God not also merciful?

#HC11

God is indeed merciful, but He is also just. His justice requires that sin committed against the most high majesty of God also be punished with the most severe, that is, with everlasting, punishment of body and soul.

Proofs

God is indeed merciful, (Exodus 20:6; Exodus 34:6–7; Psalm 103:8–9)
but He is also just. (Exodus 20:5; Exodus 34:7; Deuteronomy 7:9–11; Psalm 5:4–6; Hebrews 10:30–31)
His justice requires that sin committed against the most high majesty of God also be punished with the most severe, that is, with everlasting, punishment of body and soul. (Matthew 25:45–46)