My question is why Allah is the most merciful?

Logical argument on that Allah must be merciful to his servant?

You said that God Almighty must have mercy on his servant.
The question is: Where did this obligation come from?
Is the master obliged to pardon his slave who committed a deliberate mistake, and the master forbade him from committing it, and warned him that if he did so, he would be punished with 10 lashes for example?
Mercy comes when the master is kind, and pardons the slave despite his intentional sin, and the entitlement to punishment is achieved.
This is the mercy of Allah.