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
Working as Intended
Here's the real command to kill all cows 20 blocks around you :
/kill @e[type=Cow,r=20]