Writing to alpha channel of sprite

I want to dynamicaly create alpha mask in the sprite. I have a one sprite with alpha = 0. Shader has two passes. The first analize coordinates and write to the alpha channel. The second read alpha if alpha of pixel = 1 then the second pass returns color of sprite else returns black color.

Problem is the alpha-channel always equal to 1.

Here is the first pass:

             #pragma enable_d3d11_debug_symbols
             #pragma target 4.0
             #pragma vertex vert
             #pragma fragment frag
 
             #include "UnityCG.cginc"
 
             struct appdata
             {
                 float4 vertex : POSITION;
                 float2 uv : TEXCOORD0;
             };
 
             struct v2f
             {
                 float2 uv : TEXCOORD0;
                 float4 vertex : SV_POSITION;
                 float2 w_pos : TEXTCOOD1;
             };
 
             sampler2D _MainTex;
             float _Radius;
             float3 _Pos;
 
             v2f vert(appdata v)
             {
                 v2f o;
                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.w_pos = mul(unity_ObjectToWorld, v.vertex).xy;
                 o.uv = v.uv;
 
                 return o;
             }
 
             struct FragOut {
                 fixed4 color : SV_TARGET;
             };
 
             FragOut frag(v2f i)
             {
                 float4 mask_col = tex2D(_MainTex, i.uv);
                 FragOut out_col;
                 fixed4 red_col = { 1, 0, 0, 1 };
                 fixed4 transparent = { 0, 0, 0, 0 };
 
                 fixed2 pos = i.w_pos;
                 pos.x -= _Pos.x;
                 pos.y -= _Pos.y;
                 out_col.color = red_col;
                 return out_col;
 
                 if (pos.x * pos.x + pos.y * pos.y < _Radius * _Radius || mask_col.a == 1.0)
                     out_col.color = red_col;
                 else
                     out_col.color = transparent;
                 return out_col;
             }

Here is the second pass:

 #pragma enable_d3d11_debug_symbols
             #pragma target 4.0
             #pragma vertex vert
             #pragma fragment frag
 
             #include "UnityCG.cginc"
 
             struct appdata
             {
                 float4 vertex : POSITION;
                 float2 uv : TEXCOORD0;
                 fixed4 color : COLOR;
             };
 
             struct v2f
             {
                 float2 uv : TEXCOORD0;
                 float4 vertex : SV_POSITION;
                 fixed4 color : COLOR;
             };
 
             sampler2D _MainTex;
 
             v2f vert(appdata v)
             {
                 v2f o;
                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.color = v.color;
                 o.uv = v.uv;
 
                 return o;
             }
 
             fixed4 frag(v2f i) : SV_Target
             {
                 fixed4 col = tex2D(_MainTex, i.uv);
                 fixed4 black_color = { 0, 0, 0, 1 };
                 fixed4 color = { 1, col.r, col.g, col.b};
 
                 if (col.a == 1.0)
                     return color;
 
                 return black_color;
             }