/kill Cow 20 or any mob it will kill you and every thing

Put the summary of the bug you're having here

What I expected to happen was...:
kill all cows in 20 blocks

What actually happened was...:
killed all mobs and entity and you

Steps to Reproduce:
1./kill
2. .../kill Cow or what mob you want
3. .../kill Cow 20 or what ever