If your execute if entity does not rely on NBT data, you can simply move the selector into the command you want to use. For example, if you have:
execute if entity @a[team=red]
You can also use:
kill @a[team=red]
If your testfor relies on NBT data, you will need to use either scoreboard players set
or scoreboard players tag
to give them a scoreboard score/tag, then select them based off of that. For example if you have:
testfor @a {OnGround:1b}
You can give them a tag like this:
scoreboard players tag @a add IsGrounded {OnGround:1b}
Then select them with @a[tag=IsGrounded]
. You’ll probably want to remove this tag afterwards, so that people who match the data once won’t always have the tag:
scoreboard players tag @a[tag=IsGrounded] remove IsGrounded
In 1.13, you can specify NBT data directly in a selector, for example:
kill @a[nbt={OnGround:1b}]