/execute doesn't work well

What I expected to happen was...:
Mobs stand on stone gets die,other Mobs doesn't gets die.
What actually happened was...:
When Mobs that doesn't stand on a stone is exist, Mobs stand on a stone doesn't die too
Commands to Reproduce:

/execute as @e[tag=example] at @s if block ~ ~-1 ~ stone run kill @s