Range class to represent limited values, like HP, Stamina and so on
Problematic
Fill and Deplete are public, but don't call UpdateSlider, whereas Increment/Decrement do.

SetMax should reject values &lt; 0 (0 could be allowed, for value forced to 0)

Design Style
There is a function in the Unity Mathf static class that does bounds checking Mathf.Clamp

C# has properties, while you can have the pair GetX/SetX they are preferred.

public int Value { 
    get { return val; } 
    set { val = Mathf.Clamp( value, 0, Max ); 
        UpdateSlider(); } }
public int Max { 
    get { return max; } 
    set { if ( value &lt; 0 ) throw new ArgumentOutOfRangeException ("Max"); 
        max = value; } }


Consider
Increment and Decrement could be replaced by overriding + and -, as += and -= will use such overloads.